Steps to reproduce the issue

Load a radio buttons list using HTMLHelper('select.radiolist') and pass a CSS class in the attributes.

$options=array();
$o = new stdClass();
$o->value = 1;
$o->text = 'option 1';
$options[] = $o;

$attribs=array();
$attribs['class'] = 'MyCLass'
HTMLHelper::_('select.radiolist', $options, 'fieldname', $attribs);

Expected result

Both the form-check-input and MyCLass should be merged in one class attribute

<div class="controls">
  <div class="form-check form-check-inline">
    <input type="radio" class="form-check-input MyCLass"  name="fieldname" id="fieldname1" value="1">
    <label for="fieldname1" class="form-check-label" id="fieldname1-lbl">option 1</label>
  </div>
</div>

Actual result

class attribute is duplicated.

<div class="controls">
  <div class="form-check form-check-inline">
    <input type="radio" class="form-check-input"  name="fieldname" id="fieldname1" value="1" class="MyCLass">
    <label for="fieldname1" class="form-check-label" id="fieldname1-lbl">option 1</label>
  </div>
</div>

Modern browser will remove the second instance of the class attribute and that can cause developper headaches...
This is in particular an issue with Virtuemart that is loading some radio list and then some script is looking for the selected option based on a passed class but the class has been removed by the browser (or some cleanup / compress html scripts)

System information (as much as possible)

Php 8.1
Joomla 4.3.3

Additional comments