diff --git a/roomserver/storage/postgres/state_snapshot_table.go b/roomserver/storage/postgres/state_snapshot_table.go
index 63175955..2a8528d5 100644
--- a/roomserver/storage/postgres/state_snapshot_table.go
+++ b/roomserver/storage/postgres/state_snapshot_table.go
@@ -46,13 +46,18 @@ CREATE TABLE IF NOT EXISTS roomserver_state_snapshots (
     -- Unused in normal operation, but useful for background work or ad-hoc debugging.
     room_nid bigint NOT NULL,
     -- List of state_block_nids, stored sorted by state_block_nid.
-    state_block_nids bigint[] NOT NULL
+    state_block_nids bigint[] NOT NULL,
+	-- Deduplicate state snapshots if we can.
+	UNIQUE (room_nid, state_block_nids)
 );
 `
 
 const insertStateSQL = "" +
 	"INSERT INTO roomserver_state_snapshots (room_nid, state_block_nids)" +
 	" VALUES ($1, $2)" +
+	" ON CONFLICT (room_nid, state_block_nids) DO UPDATE SET room_nid=$1" +
+	// Performing an update, above, ensures that the RETURNING statement
+	// below will always return a valid state snapshot ID
 	" RETURNING state_snapshot_nid"
 
 // Bulk state data NID lookup.
@@ -87,7 +92,12 @@ func (s *stateSnapshotStatements) InsertState(
 	for i := range stateBlockNIDs {
 		nids[i] = int64(stateBlockNIDs[i])
 	}
-	err = sqlutil.TxStmt(txn, s.insertStateStmt).QueryRowContext(ctx, int64(roomNID), pq.Int64Array(nids)).Scan(&stateNID)
+	var id int64
+	err = sqlutil.TxStmt(txn, s.insertStateStmt).QueryRowContext(ctx, int64(roomNID), pq.Int64Array(nids), int64(roomNID)).Scan(&id)
+	if err != nil {
+		return 0, err
+	}
+	stateNID = types.StateSnapshotNID(id)
 	return
 }
 
diff --git a/roomserver/storage/sqlite3/state_snapshot_table.go b/roomserver/storage/sqlite3/state_snapshot_table.go
index bf49f62c..99e69113 100644
--- a/roomserver/storage/sqlite3/state_snapshot_table.go
+++ b/roomserver/storage/sqlite3/state_snapshot_table.go
@@ -33,13 +33,17 @@ const stateSnapshotSchema = `
   CREATE TABLE IF NOT EXISTS roomserver_state_snapshots (
     state_snapshot_nid INTEGER PRIMARY KEY AUTOINCREMENT,
     room_nid INTEGER NOT NULL,
-    state_block_nids TEXT NOT NULL DEFAULT '[]'
+    state_block_nids TEXT NOT NULL DEFAULT '[]',
+	UNIQUE (room_nid, state_block_nids)
   );
 `
 
 const insertStateSQL = `
 	INSERT INTO roomserver_state_snapshots (room_nid, state_block_nids)
-	  VALUES ($1, $2);`
+	  VALUES ($1, $2)
+	  ON CONFLICT (room_nid, state_block_nids) DO UPDATE SET room_nid=$3
+	  RETURNING state_snapshot_nid
+`
 
 // Bulk state data NID lookup.
 // Sorting by state_snapshot_nid means we can use binary search over the result
@@ -77,15 +81,12 @@ func (s *stateSnapshotStatements) InsertState(
 		return
 	}
 	insertStmt := sqlutil.TxStmt(txn, s.insertStateStmt)
-	res, err := insertStmt.ExecContext(ctx, int64(roomNID), string(stateBlockNIDsJSON))
+	var id int64
+	err = insertStmt.QueryRowContext(ctx, int64(roomNID), string(stateBlockNIDsJSON), int64(roomNID)).Scan(&id)
 	if err != nil {
 		return 0, err
 	}
-	lastRowID, err := res.LastInsertId()
-	if err != nil {
-		return 0, err
-	}
-	stateNID = types.StateSnapshotNID(lastRowID)
+	stateNID = types.StateSnapshotNID(id)
 	return
 }