Code modification to avoid duplicate node entry in subqueue

Hi,
I suggest a little code modification to avoid several same node entry in the same subqueue.
At this moment, when you hit the ajax auto-complete add button in the admin interface, the node will be duplicated in the nodes list in the subqueue and it's only when you hit the save button than the duplicate node will be deleted.
It provide some problems when you add the node by the ajax menu in the nodequeue tab panel.
I agree than this case is rare but it happens to us.
Example : someone modify the nodequeue in the admin interface without saving (the user make all the changes before saving). During this time, another user make a modification in the queue (through nodequeue tab panel) by adding a node that is already in the node list modified by the first user. The result is than you got twice the same node in the queue list.
Patch file will be attached.