2 files changed, 4 insertions, 2 deletions
diff --git a/synapse/storage/__init__.py b/synapse/storage/__init__.py
index f8d895082d..2db2e9720f 100644
--- a/synapse/storage/__init__.py
+++ b/synapse/storage/__init__.py
@@ -220,7 +220,8 @@ class DataStore(RoomMemberStore, RoomStore,
             room_id=event.room_id,
         )
 
-        self._store_state_groups_txn(txn, event, context)
+        if not outlier:
+            self._store_state_groups_txn(txn, event, context)
 
         if current_state:
             txn.execute(
diff --git a/synapse/storage/schema/state.sql b/synapse/storage/schema/state.sql
index 44f7aafb27..2c48d6daca 100644
--- a/synapse/storage/schema/state.sql
+++ b/synapse/storage/schema/state.sql
@@ -29,7 +29,8 @@ CREATE TABLE IF NOT EXISTS state_groups_state(
 
 CREATE TABLE IF NOT EXISTS event_to_state_groups(
     event_id TEXT NOT NULL,
-    state_group INTEGER NOT NULL
+    state_group INTEGER NOT NULL,
+    CONSTRAINT event_to_state_groups_uniq UNIQUE (event_id)
 );
 
 CREATE INDEX IF NOT EXISTS state_groups_id ON state_groups(id);