a/21835782/1636522

by wared

HTML

<div><span class="c1">1</span></div>
<div><span class="c1">2</span></div>
<div><span class="c2">3</span></div>
<div><span class="c1">4</span></div>
<div><span class="c2">5</span></div>
<div><span class="c3">6</span></div>

JavaScript

(function deldup(list, cls) {
    var item = list.shift();
    if (!cls[item.className]) {
        cls[item.className] = true;
    } else {
        item = item.parentNode;
        item.parentNode.removeChild(item);
    }
    if (list.length) {
        deldup(list, cls);
    }
})(
    Array.prototype.slice.call(
        document.getElementsByTagName('span')
    ), {}
);