diff --git a/src/background_data_service.py b/src/background_data_service.py
index 99e101a4..b3838322 100644
--- a/src/background_data_service.py
+++ b/src/background_data_service.py
@@ -14,11 +14,12 @@ Key Features:
 - Memory-efficient data storage
 """
 
+import itertools
 import time
 import logging
 import threading
 import requests
-from typing import Dict, Any, Optional, Callable
+from typing import Dict, Any, Optional, Callable, List
 from dataclasses import dataclass, field
 from enum import Enum
 import queue
@@ -50,8 +51,19 @@ class FetchRequest:
     max_retries: int = 3
     priority: int = 1  # Higher number = higher priority
     callback: Optional[Callable] = None
+    # Callbacks from submitters that JOINED this fetch instead of starting a
+    # duplicate one. The primary `callback` above belongs to whoever created
+    # the request; these belong to everyone who asked for the same cache_key
+    # while it was still in flight.
+    extra_callbacks: List[Callable] = field(default_factory=list)
     created_at: float = field(default_factory=time.time)
     status: FetchStatus = FetchStatus.PENDING
+    # Set once the worker has decided this response will be cached, while it
+    # still holds the lock. From that point cancelling is refused: the write
+    # is already authorised, and abandoning it here would put the payload in
+    # the cache with the callbacks suppressed -- joiners waiting forever for a
+    # fetch that did, in fact, succeed.
+    commit_claimed: bool = False
     result: Optional[Any] = None
     error: Optional[str] = None
 
@@ -74,6 +86,10 @@ class FetchResult:
     fetch_time: float = 0.0
     retry_count: int = 0
     completed_at: float = field(default_factory=time.time)  # Timestamp when request completed
+    # The request's final status, recorded so a finished request can still be
+    # reported accurately. Without it a caller can only be told COMPLETED or
+    # FAILED, which turns "you cancelled this" into "this errored".
+    final_status: Optional[FetchStatus] = None
 
 class BackgroundDataService:
     """
@@ -98,6 +114,20 @@ class BackgroundDataService:
         
         # Thread management
         self.executor = ThreadPoolExecutor(max_workers=max_workers, thread_name_prefix="BackgroundData")
+        # cache_key -> request_id for fetches currently in flight. Submitting
+        # the same key twice used to start two identical fetches: request_id
+        # carries a millisecond timestamp, so every submit looked new, and
+        # active_requests is keyed by it rather than by what is being fetched.
+        # On a real board the season-schedule key is requested by both the
+        # Recent and the Upcoming manager, which miss the cache in the same
+        # millisecond and each download and parse the same payload.
+        self._inflight_by_cache_key: Dict[str, str] = {}
+        # request_id was sport_year_milliseconds, which is not unique: two
+        # submits inside the same millisecond produced the SAME id, so one
+        # silently replaced the other in active_requests and completed_requests.
+        # Rare before, but dedupe hands this id back to every joiner as their
+        # handle for get_result(), so it has to be unique. A counter is enough.
+        self._request_seq = itertools.count()
         self.active_requests: Dict[str, FetchRequest] = {}
         self.completed_requests: Dict[str, FetchResult] = {}
         self.request_queue = queue.PriorityQueue()
@@ -185,7 +215,9 @@ class BackgroundDataService:
         if cache_key is None:
             cache_key = self.get_sport_cache_key(sport)
         
-        request_id = f"{sport}_{year}_{int(time.time() * 1000)}"
+        with self._lock:
+            request_id = (f"{sport}_{year}_{int(time.time() * 1000)}"
+                          f"_{next(self._request_seq)}")
         
         # Check cache first
         cached_data = self.cache_manager.get(cache_key)
@@ -231,7 +263,29 @@ class BackgroundDataService:
         )
         
         with self._lock:
+            existing_id = self._inflight_by_cache_key.get(cache_key)
+            existing = self.active_requests.get(existing_id) if existing_id else None
+            if existing_id and existing is None:
+                # Stranded index entry: the request it names is gone. Drop it and
+                # fetch normally. Looking the request up rather than trusting the
+                # id is what stops a stale entry wedging a key forever.
+                del self._inflight_by_cache_key[cache_key]
+            if existing is not None:
+                # Someone is already fetching this key. Ride along rather than
+                # duplicating the download, the parse and the resident copy.
+                if callback:
+                    existing.extra_callbacks.append(callback)
+                self.stats['deduplicated_requests'] = (
+                    self.stats.get('deduplicated_requests', 0) + 1
+                )
+                logger.info(
+                    "Joined in-flight fetch %s for %s (cache_key=%s) instead of "
+                    "starting a duplicate", existing_id, sport, cache_key
+                )
+                return existing_id
+
             self.active_requests[request_id] = request
+            self._inflight_by_cache_key[cache_key] = request_id
             self.stats['total_requests'] += 1
             self.stats['cache_misses'] += 1
         
@@ -256,7 +310,31 @@ class BackgroundDataService:
         
         try:
             with self._lock:
-                request.status = FetchStatus.IN_PROGRESS
+                # A request cancelled while it sat in the executor queue must
+                # stay cancelled. Overwriting the status here undid the cancel
+                # outright: the worker went on to download, cache and call back
+                # for work the caller had already withdrawn.
+                if request.status == FetchStatus.CANCELLED:
+                    cancelled_before_start = True
+                else:
+                    cancelled_before_start = False
+                    request.status = FetchStatus.IN_PROGRESS
+            if cancelled_before_start:
+                logger.info(
+                    "Request %s was cancelled before its worker started; "
+                    "skipping the fetch entirely", request.id
+                )
+                # Assign before returning: the finally block stores `result`
+                # in completed_requests, so building a fresh one here would
+                # file the untouched placeholder instead of this outcome.
+                result = FetchResult(
+                    request_id=request.id,
+                    success=False,
+                    error="cancelled",
+                    fetch_time=time.time() - start_time,
+                    retry_count=request.retry_count
+                )
+                return result
             
             logger.info(f"Starting background fetch for {request.sport} {request.year}")
             
@@ -282,6 +360,37 @@ class BackgroundDataService:
             # Log data validation
             logger.debug(f"Validated {len(events)} events for {request.sport} {request.year}")
             
+            # A cancelled request must not commit anything. Cancelling
+            # releases the cache_key, so a replacement fetch for the same key
+            # may already be in flight or finished -- writing this response to
+            # the cache now would overwrite fresher data with the response
+            # nobody wanted. The worker has no way to abort the HTTP call, so
+            # this is where the work gets discarded.
+            with self._lock:
+                cancelled = request.status == FetchStatus.CANCELLED
+                if not cancelled:
+                    # Claim the commit in the same critical section that read
+                    # the status, so a cancel cannot slip in between the check
+                    # and the cache write below. The write itself stays outside
+                    # the lock: it serialises a multi-megabyte payload to the
+                    # SD card, and holding the service lock across that would
+                    # stall every submit, status query and cancel behind it.
+                    request.commit_claimed = True
+            if cancelled:
+                logger.info(
+                    "Discarding response for cancelled request %s; %s may "
+                    "already belong to a replacement fetch",
+                    request.id, request.cache_key
+                )
+                result = FetchResult(
+                    request_id=request.id,
+                    success=False,
+                    error="cancelled",
+                    fetch_time=time.time() - start_time,
+                    retry_count=request.retry_count
+                )
+                return result
+
             # Cache the data
             self.cache_manager.set(request.cache_key, data)
             
@@ -307,7 +416,12 @@ class BackgroundDataService:
             logger.error(f"Failed to fetch {request.sport} {request.year} data: {error_msg}")
             
             with self._lock:
-                request.status = FetchStatus.FAILED
+                # Don't relabel a cancelled request. The callback gate in the
+                # finally block only suppresses CANCELLED, so promoting it to
+                # FAILED here delivered an error callback for a fetch nobody
+                # was waiting on any more.
+                if request.status != FetchStatus.CANCELLED:
+                    request.status = FetchStatus.FAILED
                 request.error = error_msg
             
             result = FetchResult(
@@ -321,9 +435,26 @@ class BackgroundDataService:
         finally:
             # Store result and clean up
             with self._lock:
+                result.final_status = request.status
                 self.completed_requests[request.id] = result
                 if request.id in self.active_requests:
                     del self.active_requests[request.id]
+                # Stop accepting joiners and take the callback list in the same
+                # critical section. A submitter that arrives after this point
+                # finds no in-flight entry and either hits the cache (written
+                # above, before the result was built) or starts a fresh fetch --
+                # what it must never do is join a fetch whose callbacks have
+                # already run and then never be called.
+                if self._inflight_by_cache_key.get(request.cache_key) == request.id:
+                    del self._inflight_by_cache_key[request.cache_key]
+                # A cancelled request delivers nothing: its joiners were told
+                # about a fetch that has been abandoned, and a replacement will
+                # call them via its own request.
+                if request.status == FetchStatus.CANCELLED:
+                    callbacks = []
+                else:
+                    callbacks = ([request.callback] if request.callback else [])
+                    callbacks.extend(request.extra_callbacks)
                 
                 # Update statistics
                 if result.success:
@@ -340,10 +471,11 @@ class BackgroundDataService:
             # Periodic cleanup after storing result
             self._cleanup_completed_requests()
             
-            # Call callback if provided
-            if request.callback:
+            # Call every callback: the original submitter's and any that joined
+            # this fetch. One raising must not stop the others being delivered.
+            for cb in callbacks:
                 try:
-                    request.callback(result)
+                    cb(result)
                 except Exception as e:
                     logger.error(f"Error in callback for request {request.id}: {e}")
                 # Delivered. Drop both references -- they point at the same
@@ -456,6 +588,8 @@ class BackgroundDataService:
                 return self.active_requests[request_id].status
             elif request_id in self.completed_requests:
                 result = self.completed_requests[request_id]
+                if result.final_status is not None:
+                    return result.final_status
                 return FetchStatus.COMPLETED if result.success else FetchStatus.FAILED
             return None
     
@@ -472,8 +606,22 @@ class BackgroundDataService:
         with self._lock:
             if request_id in self.active_requests:
                 request = self.active_requests[request_id]
+                if request.commit_claimed:
+                    # Too late: the worker holds an authorised commit. Report
+                    # the failure rather than half-cancelling a request whose
+                    # data is about to land in the cache.
+                    logger.debug(
+                        "Not cancelling %s: its response is already being "
+                        "committed", request_id
+                    )
+                    return False
                 request.status = FetchStatus.CANCELLED
                 del self.active_requests[request_id]
+                # Cancelling is the other way a request leaves active_requests,
+                # so the in-flight index has to be released here too or the key
+                # stays pointed at a request that no longer exists.
+                if self._inflight_by_cache_key.get(request.cache_key) == request_id:
+                    del self._inflight_by_cache_key[request.cache_key]
                 logger.info(f"Cancelled request {request_id}")
                 return True
             return False
diff --git a/test/test_background_fetch_dedupe.py b/test/test_background_fetch_dedupe.py
new file mode 100644
index 00000000..4398a474
--- /dev/null
+++ b/test/test_background_fetch_dedupe.py
@@ -0,0 +1,450 @@
+"""A second request for a key already being fetched must join, not duplicate.
+
+request_id embeds a millisecond timestamp and active_requests is keyed by it,
+so every submit looked new and nothing compared what was actually being
+fetched. On a real board the season-schedule cache_key is requested by both
+the Recent and the Upcoming manager: they miss the cache in the same
+millisecond and each start a full download and parse of the same payload.
+Measured on a running board, 138 background fetches in 24 hours arriving in
+pairs at identical timestamps -- half of them redundant.
+
+The cost of a duplicate is a second download, a second JSON parse (the
+expensive part on a Pi), and a second parsed copy resident at the same time.
+Schedules on that board run from 256KB to 20MB. It also consumes a second of
+the three executor slots with identical work, which is what makes two large
+parses peak simultaneously.
+"""
+
+import threading
+import time
+from unittest.mock import MagicMock, Mock, patch
+
+import pytest
+import requests
+
+from src.background_data_service import BackgroundDataService, FetchStatus
+
+
+PAYLOAD = {"events": [{"id": f"g{i}"} for i in range(20)]}
+
+
+@pytest.fixture
+def cache():
+    m = MagicMock()
+    m.get.return_value = None          # always a miss: force the fetch path
+    m.set.return_value = None
+    return m
+
+
+@pytest.fixture
+def service(cache):
+    svc = BackgroundDataService(cache, max_workers=3, request_timeout=5)
+    yield svc
+    svc.shutdown(wait=False)
+
+
+def _resp():
+    r = Mock()
+    r.json.return_value = PAYLOAD
+    r.raise_for_status.return_value = None
+    return r
+
+
+def _wait(service, req_id, timeout=5):
+    deadline = time.time() + timeout
+    while not service.is_request_complete(req_id) and time.time() < deadline:
+        time.sleep(0.02)
+
+
+class _BlockingSession:
+    """Holds the first fetch open so a second can be submitted mid-flight."""
+
+    def __init__(self):
+        self.calls = 0
+        self.release = threading.Event()
+        self.started = threading.Event()
+
+    def get(self, *a, **k):
+        self.calls += 1
+        self.started.set()
+        self.release.wait(timeout=5)
+        return _resp()
+
+
+def test_a_second_submit_for_the_same_key_does_not_fetch_twice(service):
+    session = _BlockingSession()
+    with patch.object(service, "session", session):
+        first = service.submit_fetch_request(
+            sport="nba", year=2026, url="https://x/s", cache_key="nba_2026",
+            callback=lambda r: None, max_retries=0)
+        assert session.started.wait(timeout=5)
+
+        second = service.submit_fetch_request(
+            sport="nba", year=2026, url="https://x/s", cache_key="nba_2026",
+            callback=lambda r: None, max_retries=0)
+
+        assert second == first, "the joiner should share the in-flight request id"
+        session.release.set()
+        _wait(service, first)
+
+    assert session.calls == 1, f"the payload was fetched {session.calls} times"
+
+
+def test_the_joiner_still_gets_its_callback(service):
+    session = _BlockingSession()
+    seen = []
+    with patch.object(service, "session", session):
+        first = service.submit_fetch_request(
+            sport="nba", year=2026, url="https://x/s", cache_key="k",
+            callback=lambda r: seen.append("first"), max_retries=0)
+        assert session.started.wait(timeout=5)
+        joined = service.submit_fetch_request(
+            sport="nba", year=2026, url="https://x/s", cache_key="k",
+            callback=lambda r: seen.append("second"), max_retries=0)
+        # Assert the coalescing happened, otherwise this passes trivially:
+        # two independent requests would each fire their own callback and the
+        # test would say nothing about the joined path.
+        assert joined == first
+        session.release.set()
+        _wait(service, first)
+
+    deadline = time.time() + 5
+    while len(seen) < 2 and time.time() < deadline:
+        time.sleep(0.02)
+    assert sorted(seen) == ["first", "second"], (
+        f"both submitters must be called back, got {seen}")
+
+
+def test_one_callback_raising_does_not_silence_the_other(service):
+    session = _BlockingSession()
+    seen = []
+
+    def boom(result):
+        raise RuntimeError("consumer blew up")
+
+    with patch.object(service, "session", session):
+        first = service.submit_fetch_request(
+            sport="nba", year=2026, url="https://x/s", cache_key="k",
+            callback=boom, max_retries=0)
+        assert session.started.wait(timeout=5)
+        joined = service.submit_fetch_request(
+            sport="nba", year=2026, url="https://x/s", cache_key="k",
+            callback=lambda r: seen.append("survivor"), max_retries=0)
+        # Same reason: without coalescing these are separate requests and
+        # neither callback can affect the other.
+        assert joined == first
+        session.release.set()
+        _wait(service, first)
+
+    deadline = time.time() + 5
+    while not seen and time.time() < deadline:
+        time.sleep(0.02)
+    assert seen == ["survivor"]
+
+
+def test_different_keys_are_not_coalesced(service):
+    session = _BlockingSession()
+    with patch.object(service, "session", session):
+        a = service.submit_fetch_request(
+            sport="nba", year=2026, url="https://x/a", cache_key="key_a",
+            callback=lambda r: None, max_retries=0)
+        assert session.started.wait(timeout=5)
+        b = service.submit_fetch_request(
+            sport="nhl", year=2026, url="https://x/b", cache_key="key_b",
+            callback=lambda r: None, max_retries=0)
+        assert a != b, "different cache keys must not share a request"
+        session.release.set()
+        _wait(service, a)
+        _wait(service, b)
+    assert session.calls == 2
+
+
+def test_a_later_submit_after_completion_fetches_again(service):
+    """Dedupe is for concurrent requests only, not a second cache layer."""
+    with patch.object(service.session, "get", side_effect=[_resp(), _resp()]) as get:
+        first = service.submit_fetch_request(
+            sport="nba", year=2026, url="https://x/s", cache_key="k",
+            callback=lambda r: None, max_retries=0)
+        _wait(service, first)
+        second = service.submit_fetch_request(
+            sport="nba", year=2026, url="https://x/s", cache_key="k",
+            callback=lambda r: None, max_retries=0)
+        _wait(service, second)
+    assert first != second
+    assert get.call_count == 2
+
+
+def test_cancelling_releases_the_key(service):
+    """A cancelled request must not wedge its key against future fetches."""
+    session = _BlockingSession()
+    with patch.object(service, "session", session):
+        first = service.submit_fetch_request(
+            sport="nba", year=2026, url="https://x/s", cache_key="k",
+            callback=lambda r: None, max_retries=0)
+        assert session.started.wait(timeout=5)
+        service.cancel_request(first)
+        assert "k" not in service._inflight_by_cache_key
+        session.release.set()
+
+
+def test_a_stranded_index_entry_cannot_wedge_a_key(service):
+    """Defensive: the request is looked up, not trusted from the id alone."""
+    service._inflight_by_cache_key["ghost"] = "no_such_request"
+    with patch.object(service.session, "get", return_value=_resp()):
+        req = service.submit_fetch_request(
+            sport="nba", year=2026, url="https://x/s", cache_key="ghost",
+            callback=lambda r: None, max_retries=0)
+        _wait(service, req)
+    assert service.get_result(req).success is True
+
+
+def test_the_deduplicated_count_is_reported(service):
+    session = _BlockingSession()
+    with patch.object(service, "session", session):
+        first = service.submit_fetch_request(
+            sport="nba", year=2026, url="https://x/s", cache_key="k",
+            callback=lambda r: None, max_retries=0)
+        assert session.started.wait(timeout=5)
+        service.submit_fetch_request(
+            sport="nba", year=2026, url="https://x/s", cache_key="k",
+            max_retries=0)
+        session.release.set()
+        _wait(service, first)
+    assert service.get_statistics().get("deduplicated_requests") == 1
+
+
+def test_a_cancelled_worker_cannot_overwrite_its_replacement(service, cache):
+    """Cancelling frees the key, so a replacement may already own it.
+
+    The worker cannot abort an HTTP call in flight, so when the cancelled one
+    finally returns it must discard its response rather than write it. Without
+    that, the sequence is: cancel A, submit B for the same key, B fetches and
+    caches fresh data, A returns and overwrites it with the response nobody
+    wanted -- and calls A's callbacks too.
+    """
+    slow = _BlockingSession()
+    stale = {"events": [{"id": "STALE"}]}
+    slow_resp = Mock()
+    slow_resp.json.return_value = stale
+    slow_resp.raise_for_status.return_value = None
+
+    def blocked_get(*a, **k):
+        slow.calls += 1
+        slow.started.set()
+        slow.release.wait(timeout=5)
+        return slow_resp
+
+    called = []
+    with patch.object(service.session, "get", side_effect=blocked_get):
+        first = service.submit_fetch_request(
+            sport="nba", year=2026, url="https://x/s", cache_key="k",
+            callback=lambda r: called.append("cancelled_one"), max_retries=0)
+        assert slow.started.wait(timeout=5)
+
+        service.cancel_request(first)
+        assert "k" not in service._inflight_by_cache_key
+
+    # The replacement writes the fresh value while the cancelled fetch is held.
+    fresh = {"events": [{"id": "FRESH"}]}
+    fresh_resp = Mock()
+    fresh_resp.json.return_value = fresh
+    fresh_resp.raise_for_status.return_value = None
+    with patch.object(service.session, "get", return_value=fresh_resp):
+        second = service.submit_fetch_request(
+            sport="nba", year=2026, url="https://x/s", cache_key="k",
+            callback=lambda r: called.append("replacement"), max_retries=0)
+        _wait(service, second)
+
+    assert cache.set.call_args[0][1] == fresh, "replacement must own the cache"
+
+    # Now let the cancelled fetch finish. It must write nothing and call nobody.
+    # Wait for the worker to actually finish rather than sleeping: a fixed
+    # sleep is a race under load, and a slow worker would make this pass for
+    # the wrong reason. A cancelled request is still filed in
+    # completed_requests, so that is the signal it has run to completion.
+    writes_before = cache.set.call_count
+    slow.release.set()
+    deadline = time.time() + 5
+    while first not in service.completed_requests and time.time() < deadline:
+        time.sleep(0.02)
+    assert first in service.completed_requests, "cancelled worker never finished"
+
+    assert cache.set.call_count == writes_before, (
+        "the cancelled worker wrote to the cache after its replacement")
+    assert cache.set.call_args[0][1] == fresh, "stale data overwrote fresh"
+    assert "cancelled_one" not in called, (
+        "a cancelled request must not deliver callbacks")
+
+
+def test_request_ids_are_unique_within_a_millisecond(service):
+    """request_id was sport_year_milliseconds, which collides.
+
+    Two submits inside the same millisecond produced the SAME id, so one
+    silently replaced the other in active_requests and completed_requests.
+    Dedupe hands this id back to every joiner as their handle for
+    get_result(), so uniqueness is now load-bearing rather than incidental.
+    """
+    # Stub the executor rather than the session: this is about what submit
+    # hands back, and letting 50 workers loose would outlive the patch and
+    # make real network calls.
+    with patch.object(service.executor, "submit"):
+        ids = [
+            service.submit_fetch_request(
+                sport="nba", year=2026, url="https://x/s",
+                cache_key=f"key_{i}",           # distinct keys: no dedupe
+                callback=lambda r: None, max_retries=0)
+            for i in range(50)
+        ]
+    assert len(set(ids)) == len(ids), "request ids collided"
+
+
+# --- cancellation must be terminal ------------------------------------------
+#
+# Cancelling used to be advisory: three separate paths wrote request.status
+# without checking whether the request had already been cancelled, so a cancel
+# could be silently undone and the work it was meant to stop went ahead.
+
+URL = "http://example.invalid/scores"
+KEY = "sched_nfl_2025"
+
+
+class _CountingSession:
+    """Records whether an HTTP fetch was ever attempted."""
+
+    def __init__(self):
+        self.calls = 0
+
+    def get(self, *a, **k):
+        self.calls += 1
+        return _resp()
+
+
+class _BlockingFailingSession(_CountingSession):
+    """Holds the fetch open, then fails it.
+
+    Cancelling while the worker is parked inside the HTTP call is the only way
+    to reach the exception handler as a cancelled request. Cancel it before
+    the call starts and the worker returns at the pre-start branch instead,
+    which would leave the except path untested.
+    """
+
+    def __init__(self):
+        super().__init__()
+        self.started = threading.Event()
+        self.release = threading.Event()
+
+    def get(self, *a, **k):
+        self.calls += 1
+        self.started.set()
+        self.release.wait(timeout=5)
+        raise requests.RequestException("connection reset")
+
+
+def _fill_every_worker_slot(service, slots=3):
+    """Occupy the pool so the next submit is queued rather than started.
+
+    This is what makes "cancel before the worker runs" deterministic instead
+    of a race the test would win only sometimes. Returns the gate that
+    releases the pool.
+    """
+    gate = threading.Event()
+    for _ in range(slots):
+        service.executor.submit(gate.wait, 5)
+    return gate
+
+
+def test_cancelling_before_the_worker_starts_stops_the_fetch(service, cache):
+    """The queued worker must honour a cancel, not overwrite it with IN_PROGRESS.
+
+    Between submit and the worker picking the job up, the request sits in the
+    executor queue. Cancelling there is the cheapest possible cancel -- nothing
+    has been downloaded yet -- and it was the one that did not work.
+    """
+    gate = _fill_every_worker_slot(service)
+    session = _CountingSession()
+    delivered = []
+
+    with patch.object(service, 'session', session):
+        rid = service.submit_fetch_request(
+            "nfl", 2025, URL, KEY, max_retries=0, callback=delivered.append
+        )
+        assert service.cancel_request(rid) is True
+        gate.set()                       # let the queued worker run
+        _wait(service, rid)
+
+    assert session.calls == 0, (
+        "cancelled before it started, yet the worker still downloaded the payload"
+    )
+    assert cache.set.call_count == 0, "a cancelled request wrote to the cache"
+    assert delivered == [], "a cancelled request invoked its callbacks"
+
+
+def test_a_cancel_during_the_commit_is_refused(service, cache):
+    """Once the worker has claimed the commit, cancelling is too late.
+
+    The claim and the cancelled-check happen in one critical section, so a
+    cancel arriving after it cannot retroactively abandon data already on its
+    way to the cache. Letting it through stranded every joiner: the payload
+    landed in the cache but the callbacks were suppressed, so a manager that
+    joined this fetch waited for a call that never came.
+    """
+    gate = _fill_every_worker_slot(service)
+    late = {}
+    delivered = []
+
+    def cancel_mid_write(key, data, *a, **k):
+        late['returned'] = service.cancel_request(late['rid'])
+
+    cache.set.side_effect = cancel_mid_write
+
+    # Read the payload inside the callback. The service releases result.data
+    # once every callback has been delivered, so inspecting the FetchResult
+    # afterwards sees the released object, not what the caller was handed.
+    def record(result):
+        delivered.append((result.success, result.data))
+
+    with patch.object(service, 'session', _CountingSession()):
+        late['rid'] = service.submit_fetch_request(
+            "nfl", 2025, URL, KEY, max_retries=0, callback=record
+        )
+        gate.set()                       # only now can the worker reach the commit
+        _wait(service, late['rid'])
+
+    assert late.get('returned') is False, (
+        "cancelled a request that had already committed"
+    )
+    assert cache.set.call_count == 1, "the commit itself was lost"
+    assert delivered == [(True, PAYLOAD)], (
+        "data reached the cache but the callbacks were suppressed -- "
+        "every joined submitter is left waiting forever"
+    )
+
+
+def test_a_failure_after_cancelling_stays_cancelled(service, cache):
+    """A cancelled request that then errors must not resurface as FAILED.
+
+    The except path overwrote CANCELLED with FAILED, and the callback gate in
+    the finally block only suppresses callbacks for CANCELLED -- so cancelling
+    a request that was about to time out delivered a spurious error callback.
+    """
+    session = _BlockingFailingSession()
+    delivered = []
+
+    with patch.object(service, 'session', session):
+        rid = service.submit_fetch_request(
+            "nfl", 2025, URL, KEY, max_retries=0, callback=delivered.append
+        )
+        # Assert the worker is inside the HTTP call before cancelling,
+        # otherwise this silently degrades into the pre-start case and the
+        # exception handler is never exercised.
+        assert session.started.wait(timeout=5)
+        assert service.cancel_request(rid) is True
+        session.release.set()
+        _wait(service, rid)
+
+    assert session.calls == 1, "the fetch never started, so nothing could fail"
+
+    assert delivered == [], "a cancelled request delivered a failure callback"
+    assert service.get_request_status(rid) is FetchStatus.CANCELLED, (
+        "a cancelled request that then errored was reported as FAILED"
+    )