Form XML string by recursively traversing an un-ordered list

I need a jQuery/JavaScript function to form an XML string from an un-ordered list

This is the HTML

<ul>

<li>

    <input class="checktree" type="checkbox"/>

    <label>PhoneNumbers</label>

    <ul>

        <li>

            <input class="checktree" type="checkbox"/>

            <label>PhoneNumber</label>

            <ul>

                <li>

                    <input class="checktree" type="checkbox"/>

                    <label>number</label>

                </li>

            </ul>

            <ul>

                <li>

                    <input class="checktree" type="checkbox"/>

                    <label>type</label>

                </li>

            </ul>

        </li>

    </ul>

</li>

</ul>

This is the code I have now

NOTE: item is the un-ordered list jQuery object; xml is a global var

function loadXml(item) {

    $(item).children('li').each(function () {

        var li = $(this);

        if (null != li) {

            var label = li.find('label');

            if (null != label && label.html() != null) {

                xml += '<' + label.html() + '>';

                var lisize = li.children("ul").length;

                for (var i = 0; i < lisize; i++) {

                    loadXml(li.children("ul").get(i));

                }

                xml += '</' + label.html() + '>';

            }

        }

    });

}

But the above code doesn't work properly. It generates duplicates. Below is an example of its output:

<PhoneNumbers><PhoneNumber><number></number><type></type></PhoneNumber></PhoneNumbers> (it would be perfect if it can stop here)

<PhoneNumber><number></number><type></type></PhoneNumber> (duplicates)

<number></number><type></type> (duplicates)

What's wrong w/ the recursion?