The code seems to be correct and I'm afraid I don't remember such a problem reported/fixed so far. I tried that with the latest Q2 2008 Service Pack version labeled 2008.2.826, but could not reproduce the described issue as well.
Just a quick question though, do you run that AjaxSetting this way or do you have the DatePicker set as AJAX initiator as well? Can you try adding RadDatePicker -> RadDatePicker AJAX setting instead and see whether that causes different control behavior? The AJAX setting should look like this:
| <telerik:AjaxSetting AjaxControlID="test">
| <telerik:AjaxUpdatedControl ControlID="test" />
Additionally, is the event fired twice when you remove the RadAjaxManager or simply disable it temporary (set EnableAJAX false
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