title: jquery droppable -> avoid multiple drops of the same object
body: <p>I have a container with different draggable -elements and there is a list of some "target" divs, where the user can drop the draggable elements.</p>
  
  <p>Example:
  Imagine, you have a list of "tags" (House,Computer,Car,..) and a list of some documents  as target (all documents are part of the div <code>&lt;div id="doclist"&gt;</code>). So the target is to assign the "tags" to the document using drag &amp; drop. By the way, every tag-Div has an unique id (<code>&lt;div id="e34a568b2"&gt;</code>)</p>
  
  <p>Code for making the "tags" draggable:</p>
  
  <pre><code>$('#taglist').find('div').draggable(
      {helper: "clone"});
  </code></pre>
  
  <p>Code for making the documents "droppable":</p>
  
  <pre><code>$('#doclist').droppable({
          drop: function( event, ui )
                         {tag=ui.draggable;
                          tag.clone().appendTo( this );
                         } });
  </code></pre>
  
  <p>Until now, this works well.
  The Problem is, that right now you can assign the same tag multiple times to same documents.
  Example: document 1 can get tag "House" 5 times, Tag "Computer" 3 times. </p>
  
  <p>My target is, that every document can have every tag only one time.</p>
  
  <p>I don't know, how to solve this problem. Right now, i thnik there are to ways:</p>
  
  <p>1.) expand the "drop" function by walking trough the DOM $(this).find... to see, if there is an element with the same id - in this case, don't clone&amp;append again. Probably this needs a lot of performance.</p>
  
  <p>2.) use the "accept" feature of the draggable widget. But i don't know how to use this at this situation.</p>
  
  <p>Thank you for any kind of help.</p>