Duplicate canonical meta tags when using Global redirect

When both Metatag and Global redirect are configured to output canonical tags, the end result is duplicate tags. For some reason the checking in metatag_html_head_alter() is only overriding core tags, but not the Global redirect one, which is added with the following code in globalredirect_init():

drupal_add_html_head_link(array(
  'rel' => 'canonical',
  'href' => url(drupal_is_front_page() ? '<front>' : $request_path, array('absolute' => TRUE, 'query' => $query_string)),
));