1 files changed, 1 insertions, 1 deletions

diff --git a/synapse/handlers/device.py b/synapse/handlers/device.py

index ac9868d810..0d6750f0e4 100644
--- a/synapse/handlers/device.py
+++ b/synapse/handlers/device.py
@@ -338,7 +338,7 @@ class DeviceHandler(BaseHandler):
                         etype, state_key = key
                         if etype != EventTypes.Member:
                             continue
-                        possibly_changed.append(state_key)
+                        possibly_changed.add(state_key)
                     break
 
             # If there has been any change in membership, include them in the