diff --git a/osu.Game/Online/RealtimeMultiplayer/StatefulMultiplayerClient.cs b/osu.Game/Online/RealtimeMultiplayer/StatefulMultiplayerClient.cs
index 6331d324a6..e8dbeda9cb 100644
--- a/osu.Game/Online/RealtimeMultiplayer/StatefulMultiplayerClient.cs
+++ b/osu.Game/Online/RealtimeMultiplayer/StatefulMultiplayerClient.cs
@@ -226,6 +226,10 @@ namespace osu.Game.Online.RealtimeMultiplayer
                 if (Room == null)
                     return;
 
+                // for sanity, ensure that there can be no duplicate users in the room user list.
+                if (Room.Users.Any(existing => existing.UserID == user.UserID))
+                    return;
+
                 Room.Users.Add(user);
 
                 RoomChanged?.Invoke();