remove duplicates from groups tables

and rename inconsistently named indexes.
Based on https://github.com/matrix-org/synapse/pull/3128 - thanks @vurpo\!

synapse/storage/schema/delta/48/group_unique_indexes.sql

@@ -0,0 +1,34 @@
+/* Copyright 2018 New Vector Ltd
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+-- remove duplicates from group_users table
+DELETE FROM group_users WHERE ctid NOT IN (
+   SELECT min(ctid) FROM group_users GROUP BY group_id, user_id
+);
+DROP INDEX groups_users_g_idx;
+CREATE UNIQUE INDEX group_users_g_idx ON group_users(group_id, user_id);
+
+-- remove duplicates from group_invites table
+DELETE FROM group_invites WHERE ctid NOT IN (
+   SELECT min(ctid) FROM group_invites GROUP BY group_id, user_id
+);
+DROP INDEX groups_invites_g_idx;
+CREATE UNIQUE INDEX group_invites_g_idx ON group_invites(group_id, user_id);
+
+-- rename other indexes to actually match their table names...
+ALTER INDEX groups_users_u_idx RENAME TO group_users_u_idx;
+ALTER INDEX groups_invites_u_idx RENAME TO group_invites_u_idx;
+ALTER INDEX groups_rooms_g_idx RENAME TO group_rooms_g_idx;
+ALTER INDEX groups_rooms_r_idx RENAME TO group_rooms_r_idx;