Check startup delayed state events for same state

If on startup there are multiple delayed state events to be sent, do not
send multiple events that target the same state key for a room.

synapse/handlers/delayed_events.py

@@ -27,6 +27,8 @@
     Dict,
     List,
     Optional,
+    Set,
+    Tuple,
 )
 
 import attr
@@ -100,11 +102,35 @@ async def _schedule_db_events() -> None:
             events, remaining_timeout_delays = await self.store.process_all_delays(
                 self._get_current_ts()
             )
-            for args in events:
+            sent_state: Set[Tuple[RoomID, EventType, StateKey]] = set()
+            for (
+                user_localpart,
+                room_id,
+                event_type,
+                state_key,
+                timestamp,
+                content,
+            ) in events:
+                if state_key is not None:
+                    state_info = (room_id, event_type, state_key)
+                    if state_info in sent_state:
+                        continue
+                else:
+                    state_info = None
                 try:
-                    await self._send_event(*args)
+                    await self._send_event(
+                        user_localpart,
+                        room_id,
+                        event_type,
+                        state_key,
+                        timestamp,
+                        content,
+                    )
+                    if state_info is not None:
+                        sent_state.add(state_info)
                 except Exception:
                     logger.exception("Failed to send delayed event on startup")
+            sent_state.clear()
 
             for delay_id, user_localpart, relative_delay in remaining_timeout_delays:
                 self._schedule(delay_id, user_localpart, relative_delay)