$(document).ready(function(){
var block = $("#clone_1");
block.after(block.clone());
}
<div class="slider-items">
<div id="clone_1" ><img src="images/1.jpg"/></div>
<div id="clone_2" ><img src="images/2.jpg"/></div>
<div id="clone_3" ><img src="images/3.jpg"/></div>
</div>
var block = $("#clone_1", "#clone_2", "#clone_3");