ethereal,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
.clone{
color: #00FFFF;
background-color: #0000CD;
}
</style>
</head>
<body>
<div class="content">
<div class="item">
<div class="item-inner">1</div>
</div>
<div class="item">
<div class="item-inner">2</div>
</div>
<div class="item">
<div class="item-inner">3</div>
</div>
</div>
<script>
document.querySelectorAll('.item-inner').forEach(el => {
let clone = el.cloneNode(true);
clone.classList.add('clone');
el.after(clone)
})
</script>
</body>
</html>