title: Extjs 4.2 TreeGrid collapse/expand node does not remove preceding node
body: <p>I have a treegrid in extjs 4.2. I load some data. let's say 1 node with some children. I expand the node, the children are there. I collapse the node and expand it again. After the second expand, the children are duplicated. And so and so on, after each expand, children are always duplicated.</p>
  
  <p>Did anyone else encounter this situation?</p>
  
  <pre><code>store.setRootNode(firstLevelNodesList);
  store.getRootNode().appendChild(firstLevelNodesList);
  store.getRootNode().expandChildren();
  </code></pre>
  
  <p>firstLevelNodesList is a list of nodes (they are built correctly). the call is made on first opening a window with the grid in it. The stragen behaviour is when I click on collapse/expand</p>