Merge pull request #41230 from nextcloud/indexFix

fix: add index on migration only if needed

core/Migrations/Version28000Date20230803221055.php

@@ -52,9 +52,10 @@ class Version28000Date20230803221055 extends SimpleMigrationStep {
 			$column = $table->getColumn('user_id');
 			$column->setNotnull(false);
 
-			$table->addIndex(['user_id', 'app_id', 'identifier'], 'tp_tasks_uid_appid_ident');
-
-			$changed = true;
+			if (!$table->hasIndex('tp_tasks_uid_appid_ident')) {
+				$table->addIndex(['user_id', 'app_id', 'identifier'], 'tp_tasks_uid_appid_ident');
+				$changed = true;
+			}
 		}
 
 		if ($changed) {