Add hasIndex check

Author: akhil1508

apps/dav/lib/Migration/Version1025Date20221114151721.php

@@ -23,8 +23,9 @@ public function changeSchema(IOutput $output, Closure $schemaClosure, array $opt
 		/** @var ISchemaWrapper $schema */
 		$schema = $schemaClosure();
 		$calendarObjectsTable = $schema->getTable('calendarobjects');
-		$calendarObjectsTable->addIndex(['classification'], 'calobj_clssfction_index');
-
+		if(!$calendarObjectsTable->hasIndex('calobj_clssfction_index')) {
+			$calendarObjectsTable->addIndex(['classification'], 'calobj_clssfction_index');
+		}
 		return $schema;
 	}