Root cause: renaming a calendar......
Which might mess up already synchronized calendars on your phone...
I manged to replicate the issue, but behavior was not consistent enough. It would not happen all the time, just sometimes... The use of a third party calendar (Business Calendar 2) on my phone may have further complicated matters, that's hard to tell afterwards.
Anyway, my fix:
- Open Google Calender in your browser
- Look for all entries belonging to the calendar with duplicate events
- Change the title of those events, for example add a symbol like '>'
- Open Google Calendar on your phone
- Delete all events in the specific calendar that still have the old name
- Change the title of the (no longer duplicated) events back
This didn't work for all entries though! Sometimes the only solution was to completely delete the event from the browser before I could remove the duplicate on the phone.