我的包含菜单出现两次drupal

Question

I have an issue with a menu in Drupal; this is why I need your help.

I want to replace the function in themplate.php

Actually :

function mymenu_menu_tree__menu_block__1($vars){ 
return '<div class="bloc-lien-footer">
            <p class="title">Liens</p>
            <ul>'.$vars['tree'].'</ul>
        </div>';}

This is what I want to do :

function mymenu_menu_tree__menu_block__1($vars){ 
include ('sites/all/themes/mytheme/templates/inc/nav.inc.php');}

The problem is that when I put my include it appears twice in front. When I write only text in <p> for example it appears once like I want.

How can I fix the double display of my include?