diff --git a/changelog.d/11994.misc b/changelog.d/11994.misc
new file mode 100644
index 0000000000..d64297dd78
--- /dev/null
+++ b/changelog.d/11994.misc
@@ -0,0 +1 @@
+Move common deduplication code down into `_auth_and_persist_outliers`.
diff --git a/synapse/handlers/federation_event.py b/synapse/handlers/federation_event.py
index 9edc7369d6..6dc27a38f3 100644
--- a/synapse/handlers/federation_event.py
+++ b/synapse/handlers/federation_event.py
@@ -419,8 +419,6 @@ class FederationEventHandler:
         Raises:
             SynapseError if the response is in some way invalid.
         """
-        event_map = {e.event_id: e for e in itertools.chain(auth_events, state)}
-
         create_event = None
         for e in auth_events:
             if (e.type, e.state_key) == (EventTypes.Create, ""):
@@ -439,11 +437,6 @@ class FederationEventHandler:
         if room_version.identifier != room_version_id:
             raise SynapseError(400, "Room version mismatch")
 
-        # filter out any events we have already seen
-        seen_remotes = await self._store.have_seen_events(room_id, event_map.keys())
-        for s in seen_remotes:
-            event_map.pop(s, None)
-
         # persist the auth chain and state events.
         #
         # any invalid events here will be marked as rejected, and we'll carry on.
@@ -455,7 +448,9 @@ class FederationEventHandler:
         # signatures right now doesn't mean that we will *never* be able to, so it
         # is premature to reject them.
         #
-        await self._auth_and_persist_outliers(room_id, event_map.values())
+        await self._auth_and_persist_outliers(
+            room_id, itertools.chain(auth_events, state)
+        )
 
         # and now persist the join event itself.
         logger.info("Peristing join-via-remote %s", event)
@@ -1245,6 +1240,16 @@ class FederationEventHandler:
         """
         event_map = {event.event_id: event for event in events}
 
+        # filter out any events we have already seen. This might happen because
+        # the events were eagerly pushed to us (eg, during a room join), or because
+        # another thread has raced against us since we decided to request the event.
+        #
+        # This is just an optimisation, so it doesn't need to be watertight - the event
+        # persister does another round of deduplication.
+        seen_remotes = await self._store.have_seen_events(room_id, event_map.keys())
+        for s in seen_remotes:
+            event_map.pop(s, None)
+
         # XXX: it might be possible to kick this process off in parallel with fetching
         # the events.
         while event_map:
@@ -1717,31 +1722,22 @@ class FederationEventHandler:
             event_id: the event for which we are lacking auth events
         """
         try:
-            remote_event_map = {
-                e.event_id: e
-                for e in await self._federation_client.get_event_auth(
-                    destination, room_id, event_id
-                )
-            }
+            remote_events = await self._federation_client.get_event_auth(
+                destination, room_id, event_id
+            )
+
         except RequestSendFailed as e1:
             # The other side isn't around or doesn't implement the
             # endpoint, so lets just bail out.
             logger.info("Failed to get event auth from remote: %s", e1)
             return
 
-        logger.info("/event_auth returned %i events", len(remote_event_map))
+        logger.info("/event_auth returned %i events", len(remote_events))
 
         # `event` may be returned, but we should not yet process it.
-        remote_event_map.pop(event_id, None)
+        remote_auth_events = (e for e in remote_events if e.event_id != event_id)
 
-        # nor should we reprocess any events we have already seen.
-        seen_remotes = await self._store.have_seen_events(
-            room_id, remote_event_map.keys()
-        )
-        for s in seen_remotes:
-            remote_event_map.pop(s, None)
-
-        await self._auth_and_persist_outliers(room_id, remote_event_map.values())
+        await self._auth_and_persist_outliers(room_id, remote_auth_events)
 
     async def _update_context_for_auth_events(
         self, event: EventBase, context: EventContext, auth_events: StateMap[EventBase]