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<script>
$(function(){
    var nowchecked = $('input[name=xxxx]:checked').val();
    $('input[name=xxxx]').click(function(){
        if($(this).val() == nowchecked) {
            $(this).prop('checked', false);
            nowchecked = false;
        } else {
            nowchecked = $(this).val();
        }
    });
});
</script>