Prevent duplicate messages

There are circumstances in which the maintenance message can appear more than once. For example, if a page triggers an AJAX event that returns JSON, the theme layer is never invoked for the AJAX request and thus drupal_get_messages() is never called, which normally clears the messages buffer. The page load subsequent such AJAX event will display the maintenance message twice.

The fix is simple: setting $repeat = FALSE for drupal_set_message() will prevent duplication.