changeExample

HTML

<table id="testTable">
  <tbody>
    <tr>
      <th>Letter</th>
      <th>Amount</th>
    </tr>
    <tr>
      <td class="parent">A</td>
      <td>110</td>
    </tr>
    <tr>
      <td class="parent">A</td>
      <td>99</td>
    </tr>
    <tr>
      <td class="parent">A</td>
      <td>106</td>
    </tr>
    <tr>
      <td class="parent">A</td>
      <td>120</td>
    </tr>
    <tr>
      <td class="parent">C</td>
      <td>103</td>
    </tr>
  </tbody>
</table>

JavaScript

$(document).ready(function(){
var lastVal = '';
$('#testTable .parent').each(function () {
        var currentVal = $(this).html();
        if(currentVal != lastVal) {
            $(this).parent().before('</tr><tr><td class="sub0">Sub Total</td></tr>');
            lastVal = currentVal;
        }
    });
});