sortable fires sortupdate-event twice

When you have an unordered encapsulated list. And you want to sort two parts of it. When you sort the nested node the sortupdate-event fires twice.

For instance you have such a list:

<ul id="menulist">
<li id="mainmenu_1" class="mainmenuitem">Start
	<ul class="submenulist">
		<li id="submenu_9">Home</li>
		<li id="submenu_7">Homer</li>
		<li id="submenu_6">Homest</li>
	</ul>
<ul>
<li id="mainmenu_2" class="mainmenuitem">Foobar
	<ul class="submenulist">
		<li id="submenu_5">Host</li>
		<li id="submenu_4">Hoster</li>
	</ul>
<ul>

"Start" and "Foobar" is the first part. In their nested nodes is the second part of the encapsulated list.

And this jquery code:

$(\\"#menulist:not(.submenulist)\\").sortable({ items: 'li.mainmenuitem' });
$(\\".submenulist\\").sortable();

$(\\"#menulist:not(.submenulist)\\").bind(\\"sortupdate\\",function(event,ui){
	var order = $(this).sortable('serialize');
	$.get(\\"menu.order.php\\",order);
});

$(\\".submenulist\\").bind(\\"sortupdate\\",function(event,ui){
	var order = $(this).sortable('serialize');
	$.get(\\"submenu.order.php\\",order);
});

For example, when you sort "hoster" above "host" the sortupdate-event of .submenulist and #menulist:not(.submenulist) are fired.

Is it a bug or feature? Or a false approach?