Dublicate all css classses and print it to dom

How can I copy all elements and dublicate it? For example:

 else if (event.keyCode == 84) {
        var arr = document.querySelectorAll('.selected');
        if(arr.classList.contains('selected')) {
            for (var i = 0; i < arr.length; i++) {
                var cln = arr[i].cloneNode(true);
                var content = document.getElementById("content");
                content.appendChild(cln);
                console.log(cln.length);
                }
        }  

    }

Solved. Problem was var cln = arr[i].cloneNode(true); after removing [i] it started to work.