Only fire user_joined_room if the membership has changed

synapse/handlers/federation.py

@@ -233,6 +233,11 @@ class FederationHandler(BaseHandler):
 
         if event.type == EventTypes.Member:
             if event.membership == Membership.JOIN:
+                context = yield self.state_handler.compute_event_context(
+                    event, old_state=state, outlier=event.internal_metadata.is_outlier()
+                )
+                prev_state = context.current_state.get((event.type, event.state_key))
+                if not prev_state or prev_state.membership != Membership.JOIN:
                     user = UserID.from_string(event.state_key)
                     yield self.distributor.fire(
                         "user_joined_room", user=user, room_id=event.room_id