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// Get Top Parent Page (in functions.php)
function get_top_parent() {
   global $post;
   if($post->post_parent) {
      $ancestors = array_reverse(get_post_ancestors($post->ID));
      return $ancestors[0];
   }
   return $post->ID;
}

// Get submenu items (insert in page submenu)
$args = array(
   'child_of' => get_top_parent(),
   'title_li' => ''
);

wp_list_pages($args);