JSFiddle - React, Tailwind, and code Playground

HTML

<select id="UnitList_Id">
<option>A</option>
<option>B</option>
<option>C</option>
</select>


<select id="SecondUnit_Id">
<option>A</option>
<option>B</option>
<option>C</option>
</select>

JavaScript

var preval=$('#SecondUnit_Id').val();
alert(preval)
$('#SecondUnit_Id').change(function(){
  //alert($('#UnitList_Id').val())
  if($(this).val() === $('#UnitList_Id').val())
    {
       alert("error Duplicate");
       $(this).val(preval);
       return false;
    
    }
    
    preval=$(this).val();
  
  });