title: jquery revert dragging if droppable already contains that element
body: <p>I am facing the dragging problem. Here's the fiddle.
  <a href="http://jsfiddle.net/dJyUQ/16/" rel="nofollow">http://jsfiddle.net/dJyUQ/16/</a></p>
  
  <p>There are 4 boxes (A, B, C, D) and all are droppables.
  ".item" div is draggable and can be dropped on any of the 4 droppables.</p>
  
  <p>I want just one copy of item to be dropped on one droppable. So that when I attempt to again drop copy of item on the same droppable it wouldn't allow me. Currently I have inserted an alert. But I want to cancel or revert dragging.</p>
  
  <p>Please help me as I think I am missing something. I searched jquery ui dragging and droppable plugin but could not find anything.</p>