unique_select_value.js

$(this).on('change', '.folder-select', function (event) {
    var $this = $(this);
    var selected_mailbox = $this.val();
    $('.folder-select').not($this).filter(function( index ) {
        return $(this).val() === selected_mailbox;
    }).each(function () {
        $(this).val('');
    });
});