JSFiddle - React, Tailwind, and code Playground

HTML

<div id="mkb">
    <select name="num" id="num" class="num" onchange="testDuplicate(this)">
        <option value="">-Select-</option>
        <option value="1">1</option>
        <option value="2">2</option>
        <option value="3">3</option>
        <option value="4">4</option>
        <option value="5">5</option>
    </select>
</div> <a href="javascript:cloneSelect()">Clone</a>

JavaScript

function cloneSelect() {
    var newSelect = jQuery("#num").clone();
    $(newSelect).val('');
    jQuery("#mkb").append(newSelect);
}

function testDuplicate(data) {
    var c = $(data).val();
    console.debug("num = " + c);
    jQuery(".num").each(function () {
        if (this != data) {
            var a = $(this).val();
            if (c == a) {
                alert("Duplicate");
                $(data).val('');
            }
        }
    });
}