JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://code.jquery.com/jquery-2.2.3.min.js"></script>
<ul class="list">
    <li class="item_20">a</li>
    <li class="item_20">b</li>
    <li class="item_20">c</li>
    <li class="item_21">d</li>
    <li class="item_21">e</li>
    <li class="item_22">f</li>
    <li class="item_22">g</li>
</ul>

JavaScript

$(document).ready(function(e) {
  var className = '';
  $('.list li').each(function(index, element) {
    if($(this).attr('class') != className){
      className = $(this).attr('class');
    }else{
      $(this).remove();
    }
  });
});