Globus,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
li.hide {
display: none;
}
</style>
<script>
document.addEventListener("DOMContentLoaded", function() {
document.querySelector("a").addEventListener("click", function(e) {
e.preventDefault();
[".clone", ".box"].forEach(function(a) {
a = document.querySelectorAll(a);
e = false;
[].forEach.call(a, function(a) {
a.classList.contains("hide") || (e ? a.classList.add("hide") : e = true)
})
})
})
});
</script>
</head>
<body>
<a href="#">Скрыть дубли</a>
<ul id="list">
<li id="item1" class="clone" >01</li>
<li id="item2" class="clone hide" >02</li>
<li id="item3" class="clone" >03</li>
<li id="item4" class="box" >04</li>
<li id="item5" class="box hide" >05</li>
<li id="item6" class="box hide" >06</li>
<li id="item7" class="box" >07</li>
</ul>
</body>
</html>