JSFiddle - React, Tailwind, and code Playground

HTML

Quarterback A
<select class="qb">
  <option>1</option>
  <option>2</option>
</select>

Quarterback B
<select class="qb">
  <option>1</option>
  <option>2</option>
</select>

Runningback A
<select class="rb">
  <option>1</option>
  <option>2</option>
</select>

Runningback B
<select class="rb">
  <option>1</option>
  <option>2</option>
</select>

<button id="eval">evaluate</button>

JavaScript

$('#eval').click(function() {
    var classes = ['qb','rb']; 
    $(classes).each(function(index, el){
        var list = [];
        $('.'+classes[index]).each(function() {
            //alert($(this).val());
            list.push($(this).val());
        });
        list = list.sort();
        for (var i = 0; i < list.length - 1; i++) {
            if (list[i + 1] == list[i]) {
                alert('duplicate ' + classes[index]);
                break;
            }
        }
    });
});