How to "dublicate" an input field in checkout page and make it check for similar input?

Code: Select all

<div class="form-group required  col-md-6">
                  <label class="control-label" for="input-payment-telephone-verify">Telephone Verification</label>
                  <input type="text" required="required" id="input-payment-telephone-verify" oninput="check(this)" class="form-control">
                  <script language="javascript" type="text/javascript">
    function check(input) {
        if (input.value != document.getElementById('input-payment-telephone').value) {
            input.setCustomValidity('Telephone Must be Matching.');
        } else {
            // input is valid -- reset the error message
            input.setCustomValidity('');
        }
    }
</script>
                </div>