Populate input with unique array of selected checkboxes with jQuery

Question

I'd like to populate an input with the comma seperated values of selected checkboxes and exclude duplicates.

The code below works but I don't know how to get the duplicates out.

HTML

<form>
    <input type="text" value="" id="tags">

    <input type="checkbox" value="apple"><label>apple</label>
    <input type="checkbox" value="banana"><label>banana</label>
    <input type="checkbox" value="melon"><label>melon</label>
    <input type="checkbox" value="melon"><label>melon</label>

</form>

jQuery

$('input[type=checkbox]').change(function() {

    var vals = $('input[type=checkbox]:checked').map(function() {
        return $(this).val();
    }).get().join(',');

    $('#tags').val(vals);

});​