diff --git a/src/backend/clients/database/SqliteDatabaseClient.test.ts b/src/backend/clients/database/SqliteDatabaseClient.test.ts
index afac01a81..715b3af8b 100644
--- a/src/backend/clients/database/SqliteDatabaseClient.test.ts
+++ b/src/backend/clients/database/SqliteDatabaseClient.test.ts
@@ -81,7 +81,23 @@ describe('SqliteDatabaseClient — boot and migrations', { timeout: DISK_MIGRATI
         expect(await userVersionOf(client)).toBe(CURRENT_SCHEMA_VERSION);
     });
 
-    it('applies the team columns and indexes from 0077', async () => {
+    it('stamps a version whose migration did not run (known off-by-one)', async () => {
+        // Existing behaviour, not an endorsement: the stamp overshoots the
+        // applied work by one entry, so 70 is reported without 0074 running.
+        const partial = await bootClient({ targetVersion: 70 });
+        try {
+            expect(await userVersionOf(partial)).toBe(70);
+            await expect(
+                partial.read(
+                    "SELECT 1 FROM pragma_table_info('group') WHERE name = 'handle'",
+                ),
+            ).resolves.toEqual([]);
+        } finally {
+            partial.onServerShutdown();
+        }
+    });
+
+    it('applies the team columns and indexes from 0076', async () => {
         const columnsOf = async (table: string) =>
             (
                 (await client.read(
diff --git a/src/backend/clients/database/migrations/mysql/mysql_mig_33.sql b/src/backend/clients/database/migrations/mysql/mysql_mig_33.sql
index caa92900f..cf4e644a0 100644
--- a/src/backend/clients/database/migrations/mysql/mysql_mig_33.sql
+++ b/src/backend/clients/database/migrations/mysql/mysql_mig_33.sql
@@ -15,19 +15,10 @@
 -- You should have received a copy of the GNU Affero General Public License
 -- along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
--- Deduplicate then constrain. See sqlite/0072 for why duplicates exist and why
--- dropping the higher-id row is lossless.
+-- Deduplicate then constrain. See sqlite/0075 for why this is lossless.
 
--- Self-join rather than `id NOT IN (SELECT MIN(id) ... FROM jct_user_group)`,
--- which mysql refuses with error 1093 -- it will not read the table it deletes
--- from in a subquery. Keeps the lowest id of each pair.
-DELETE dup FROM `jct_user_group` dup
-JOIN `jct_user_group` keep
-  ON  dup.`user_id`  = keep.`user_id`
-  AND dup.`group_id` = keep.`group_id`
-  AND dup.`id`       > keep.`id`;
-
--- Guarded because mysql tracks no applied-state per file, so this may re-run.
+-- Both steps sit behind the index check: mysql re-runs this file on every boot, and
+-- guarding them together stops a rolling deploy deleting and indexing concurrently.
 DROP PROCEDURE IF EXISTS _puter_add_membership_pair_index;
 
 DELIMITER //
@@ -39,6 +30,13 @@ BEGIN
       AND TABLE_NAME = 'jct_user_group'
       AND INDEX_NAME = 'idx_jct_user_group_pair'
   ) THEN
+    -- Self-join because mysql refuses `NOT IN (SELECT ... FROM jct_user_group)` (1093).
+    DELETE dup FROM `jct_user_group` dup
+    JOIN `jct_user_group` keep
+      ON  dup.`user_id`  = keep.`user_id`
+      AND dup.`group_id` = keep.`group_id`
+      AND dup.`id`       > keep.`id`;
+
     ALTER TABLE `jct_user_group`
       ADD UNIQUE INDEX `idx_jct_user_group_pair` (`user_id`, `group_id`);
   END IF;
diff --git a/src/backend/clients/database/migrations/postgres/postgres_mig_22.sql b/src/backend/clients/database/migrations/postgres/postgres_mig_22.sql
index 58a9a51a9..e15c6e9f2 100644
--- a/src/backend/clients/database/migrations/postgres/postgres_mig_22.sql
+++ b/src/backend/clients/database/migrations/postgres/postgres_mig_22.sql
@@ -15,14 +15,20 @@
 -- You should have received a copy of the GNU Affero General Public License
 -- along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
--- Deduplicate then constrain. See sqlite/0072 for why duplicates exist and why
--- dropping the higher-id row is lossless.
--- Idempotent: the delete is a no-op once unique, and the index guards itself.
+-- Deduplicate then constrain. See sqlite/0075 for why this is lossless.
 
-DELETE FROM jct_user_group
-WHERE id NOT IN (
-    SELECT MIN(id) FROM jct_user_group GROUP BY user_id, group_id
-);
+-- Guarded because postgres re-runs every file on each boot; `to_regclass` follows
+-- search_path, so it resolves under a test schema too.
+DO $mig15$
+BEGIN
+    IF to_regclass('idx_jct_user_group_pair') IS NULL THEN
+        DELETE FROM jct_user_group
+        WHERE id NOT IN (
+            SELECT MIN(id) FROM jct_user_group GROUP BY user_id, group_id
+        );
+    END IF;
+END
+$mig15$;
 
 CREATE UNIQUE INDEX IF NOT EXISTS idx_jct_user_group_pair
     ON jct_user_group (user_id, group_id);
diff --git a/src/backend/clients/database/migrations/sqlite/0078_jct-user-group-pair-unique.sql b/src/backend/clients/database/migrations/sqlite/0078_jct-user-group-pair-unique.sql
index baf09da1f..3d25db275 100644
--- a/src/backend/clients/database/migrations/sqlite/0078_jct-user-group-pair-unique.sql
+++ b/src/backend/clients/database/migrations/sqlite/0078_jct-user-group-pair-unique.sql
@@ -15,17 +15,11 @@
 -- You should have received a copy of the GNU Affero General Public License
 -- along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
--- `GroupStore.addUsers` is an INSERT ... SELECT with no conflict clause, so a
--- repeated call inserts a second row for the same (user_id, group_id).
---
--- Each duplicate multiplies every row `readUserGroupPerms` returns, because it
--- joins this table on group_id alone -- two membership rows means every group
--- permission is reported twice. The index below stops that recurring; the
--- delete clears what already accumulated.
---
--- Dropping the higher-id row discards nothing: `addUsers` writes only the two
--- id columns, so `extra` and `metadata` are NULL on every row here, nothing has
--- a foreign key to `jct_user_group.id`, and no code reads it.
+-- `GroupStore.addUsers` had no conflict clause, so a repeat call duplicated the pair.
+-- `readUserGroupPerms` joins this table on group_id alone, so each duplicate reported
+-- every group permission an extra time. The delete clears what accumulated.
+-- Dropping the higher id is lossless: `addUsers` writes only the two id columns, and
+-- nothing references `jct_user_group.id`.
 
 DELETE FROM `jct_user_group`
 WHERE `id` NOT IN (
diff --git a/src/backend/stores/group/GroupStore.ts b/src/backend/stores/group/GroupStore.ts
index 5a3d1e36e..5b8c9426c 100644
--- a/src/backend/stores/group/GroupStore.ts
+++ b/src/backend/stores/group/GroupStore.ts
@@ -40,11 +40,8 @@ export class GroupStore extends PuterStore {
     async addUsers(uid: string, usernames: string[]): Promise<void> {
         if (usernames.length === 0) return;
         const placeholders = `(${usernames.map(() => '?').join(', ')})`;
-        // Ignore conflicts on the (user_id, group_id) unique index added in
-        // 0072. Re-adding a member was previously a silent duplicate row, which
-        // is what that index exists to stop -- without this it becomes a raised
-        // error instead, and every caller here treats a throw as a failed
-        // signup step worth warning about.
+        // Ignore conflicts on the unique pair index from 0072; re-adding a member
+        // was a duplicate row before it, and would raise without this.
         await this.clients.db.write(
             `${this.clients.db.insertIgnoreInto('jct_user_group')} ` +
                 '(`user_id`, `group_id`) ' +