diff --git a/SETUP/db_schema.sql b/SETUP/db_schema.sql
index e0dd47992..19b89bac6 100644
--- a/SETUP/db_schema.sql
+++ b/SETUP/db_schema.sql
@@ -655,7 +655,7 @@ CREATE TABLE `users` (
   `navbar_activity_menu` tinyint NOT NULL DEFAULT '1',
   PRIMARY KEY (`username`),
   UNIQUE KEY `api_key` (`api_key`),
-  KEY `u_id` (`u_id`),
+  UNIQUE KEY `u_id` (`u_id`),
   KEY `last_login` (`last_login`),
   KEY `t_last_activity` (`t_last_activity`),
   KEY `api_key_username` (`api_key`,`username`),
diff --git a/SETUP/upgrade/25/20260603_make_users_u_id_unique.php b/SETUP/upgrade/25/20260603_make_users_u_id_unique.php
new file mode 100644
index 000000000..c429bfaeb
--- /dev/null
+++ b/SETUP/upgrade/25/20260603_make_users_u_id_unique.php
@@ -0,0 +1,22 @@
+<?php
+$relPath = '../../../pinc/';
+include_once($relPath.'base.inc');
+
+header('Content-type: text/plain');
+
+// ------------------------------------------------------------
+
+echo "Making u_id INDEX in users UNIQUE..\n";
+$sql = "
+    ALTER TABLE users
+        DROP INDEX u_id,
+        ADD UNIQUE INDEX u_id (u_id);
+";
+
+echo "$sql\n";
+
+mysqli_query(DPDatabase::get_connection(), $sql) or die(mysqli_error(DPDatabase::get_connection()));
+
+// ------------------------------------------------------------
+
+echo "\nDone!\n";