Admin Menu display issue

I gave every <li> element an unique id, which made the main nav <li> close. When I tried to give the subnav <li> elements the same id as the main nav <li> it worked like a charm!

<ul id="menu">
    <li id="mnu_dashboard"><?=anchor('admin',$this->lang->line('backendpro_dashboard'),array('class'=>'icon_house'))?></li>
    <?php if(check('Logboek',NULL,FALSE)):?>
    <li id="mnu_uren"><?=anchor('logboek/admin/logboek','Urenregistratie',array('class'=>'icon_log'))?></li>
    <?php endif;?>
    <?php if(check('Contents',NULL,FALSE)):?>
    <li id="mnu_contents"><span class="icon_contents">Contents</span>
        <ul>
            <?php if(check('Nieuws',NULL,FALSE)):?><li id="mnu_contents"><?=anchor('contents/admin/nieuws',"Nieuws",array('class'=>'icon_nieuws'))?></li><?php endif;?>  
            <?php if(check('Portfolio',NULL,FALSE)):?><li id="mnu_contents"><?=anchor('contents/admin/portfolio',"Portfolio",array('class'=>'icon_shield'))?></li><?php endif;?>  
        </ul>
    </li>
    <?php endif;?>

    <?php if(check('System',NULL,FALSE)):?>
    
    <li id="mnu_system"><span class="icon_computer"><?=$this->lang->line('backendpro_system')?></span>
        <ul>
            <?php if(check('Members',NULL,FALSE)):?><li id="mnu_system"><?=anchor('auth/admin/members',$this->lang->line('backendpro_members'),array('class'=>'icon_group'))?></li><?php endif;?>  
            <?php if(check('Access Control',NULL,FALSE)):?><li id="mnu_system"><?=anchor('auth/admin/access_control',$this->lang->line('backendpro_access_control'),array('class'=>'icon_shield'))?></li><?php endif;?>  
            <?php if(check('Settings',NULL,FALSE)):?><li id="mnu_system"><?=anchor('admin/settings',$this->lang->line('backendpro_settings'),array('class'=>'icon_cog'))?></li><?php endif;?>  
        </ul>
    </li>
    <?php endif;?>
</ul>

The origonal code didn't assign any ID's to the LI's, but assigning non unique ID's makes code invalid.

It does not work by duplicating the ID’s. It only keeps the menu closed. When I open a subitem in the System menu, it closes the system menu.

(In [220]) The BackendPro tree menu in the admin panel now saves its open/closes status rather than it being lost between different pages, Closes #15,#45