BeforeItemMove firing twice

Im registering for MAPIFolderEvents_12_BeforeItemMoveEventHandler events on
Calendar Folder. When an appointment is deleted, or shift-deleted, my
handler does get called.

However, if I shift-delete an appointment (ie MoveTo folder in the callback
is null), and I cancel the event .. ie
public void CbkBefMov(object Item, Outlook.MAPIFolder MoveTo, ref bool
Cancel)
{
if (MoveTo == null) { Cancel = true; }
}
then the callback gets invoked a second time. This is bizzare. Any ideas
why?

Thanks
Manish Rao