If you post a forum thread and a support ticket about the same thing, please notify about this in both places.
The observed behavior is normal, because information about the RadListBox items is kept in the ViewState and the whole (updated) ViewState is returned after AJAX requests.
In addition, if the RadListBox is updated via AJAX as well, the updated information about its items will be returned in its initialization script.
You can optimize the ViewState and initialization script volume with regard to the RadListBox control by adding the items client-side. I have sent an example in the support ticket.
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