diff --git a/phpBB/install/database_update.php b/phpBB/install/database_update.php
index ab22c803ee..41983400fc 100644
--- a/phpBB/install/database_update.php
+++ b/phpBB/install/database_update.php
@@ -677,6 +677,7 @@ function _add_modules($modules_to_install)
 				WHERE module_class = '" . $db->sql_escape($module_data['class']) . "'
 					AND parent_id = {$parent_id}
 					AND left_id BETWEEN {$first_left_id} AND {$module_row['left_id']}
+				GROUP BY left_id
 				ORDER BY left_id";
 			$result = $db->sql_query($sql);
 			$steps = (int) $db->sql_fetchfield('num_modules');