From f2690c64230557701692e16264f2e65a21edab4d Mon Sep 17 00:00:00 2001
From: Mark Haines <mark.haines@matrix.org>
Date: Fri, 2 Sep 2016 19:23:22 +0100
Subject: [PATCH] Fix membership changes to be idempotent

---
 synapse/handlers/room_member.py | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index 3ba5335af7..ba49075a20 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -85,6 +85,12 @@ class RoomMemberHandler(BaseHandler):
             prev_event_ids=prev_event_ids,
         )
 
+        # Check if this event matches the previous membership event for the user.
+        duplicate = yield msg_handler.deduplicate_state_event(event, context)
+        if duplicate is not None:
+            # Discard the new event since this membership change is a no-op.
+            return
+
         yield msg_handler.handle_new_client_event(
             requester,
             event,