in a caldav system any event has a unique id. If a user has a lot of "sub"-calendars in his list, but only watching some of the subcalendars, an event accepted by another device with the same user but different set of viewed subcalendars, leads to a situation where the (here) first device can accept the event a seconded time.
As this is natural behaviour for a caldav system due to the uniqueness of the ID, a "Private Copy" function OR EVEN BETTER, "Copy to .... Calendar[cars,booking,hotels,company....]" is required to make effective Groupsharing of Calendars working. This contextmenu entry should be in the "This mail contains an already accpected event" message, same as in the contextmenu for any event in the calendar tab.
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