Selecting an item via drag can cause duplicates

[NateEag]

Select a few items by double-clicking.

Select an item by dragging it into the selected list.

Remove one of the items you added earlier by double-clicking on it.

It will be removed from the 'selected' list, and four copies of it will be added to the 'available' list. If you click the - icon instead, only two copies of it will be added to the 'available' list.

I've verified this on Safari 5.0.5, Firefox 5.0.1, and Chrome 12.0.742.122 for OS X.

This is in the master branch.

I have a patch for this, and should be issuing a pull request soon.