Prevent duplicate room IDs in m.direct

synapse/handlers/room_member.py

@@ -248,8 +248,7 @@ class RoomMemberHandler(object):
 
                     # Check which key this room is under
                     for key, room_id_list in direct_rooms.items():
-                        for rid in room_id_list:
-                            if rid == old_room_id:
+                        if old_room_id in room_id_list and room_id not in room_id_list:
                             # Add new room_id to this key
                             direct_rooms[key].append(room_id)