If there is an event in the open invitations calendar already, and the event is then accepted using the mail,
you first have to wait for the search to update so the open invitations event disappears, and then you have to disable/enable the regular calendar,
so the original event (that is now accepted) appears in the calendar.
It should update automatically.
The main event should probably override the open invitation as soon as it becomes available. (or we should be able to display the same even from multiple locations simultaneously)