Draggable element covers another one when moving back
<p>Currently I'm having a small problem with the demo at <a href="http://jsfiddle.net/nivea75ml/yCnh5/" rel="nofollow">http://jsfiddle.net/nivea75ml/yCnh5/</a>.
Whenever I drag a pink box from the list to the grey area and later move it back, it covers another one in the same list. </p>

<p>How can this behaviour be avoided?</p>
