Fix issue with newly joined room detection

It is possible that the latest state event retrieved is the very same
join event present in the membership changes being processed, in this
case we treat this as a new join.

synapse/handlers/sync.py

@@ -1765,7 +1765,9 @@ async def _get_rooms_changed(
                 old_state_ids = await self.get_state_at(room_id, since_token)
                 old_mem_ev_id = old_state_ids.get((EventTypes.Member, user_id), None)
                 old_mem_ev = None
-                if old_mem_ev_id:
+                # It is possible that the old membership event is the same as the
+                # latest change event we are handling, see `self.get_state_at`.
+                if old_mem_ev_id and old_mem_ev_id != events[-1].event_id:
                     old_mem_ev = await self.store.get_event(
                         old_mem_ev_id, allow_none=True
                     )