checkboxes return both 'true' and 'false' with jquery serialize()

In my form I have a couple of checkboxes. Here's one:

<input 
    checked="checked" 
    class="check-box valid" 
    data-val="true" 
    data-val-required="The MustBePaidOff field is required."
    id="MustBePaidOff"
    name="MustBePaidOff"
    type="checkbox"
    value="true">

The problem is, when I do this:

$('#SaveCriteriaForm').on('submit', function (e) {
    e.preventDefault();
    var formData = $(this).serialize();
    ....
});

Inside formData I get this:

MustBePaidOff=true&MustBePaidOff=false

What do I need to do that I get only a true or a false depending on if the checkbox is checked?