sorry, but I do not know how to reproduce that - otherwise I would do my best to avoid it. Just arbitrary it happens sometimes. Maybe it is caused by a bad connection so that the sync cannot be finished.
Server: Horde 5.2.13 with Kronolith H5 (4.2.19)
logfile is difficult cause I cannot provoke the problem
But … on the other hand: would it be too complicated to add a button to check for dupes? Hopefully someone else comes around the corner who has the same problem at times. The solution of the problem is rather complicated, having to resync the whole calender, not to mention the loss of entries older than the set time (that are not synced any more but stay on the device nevertheless)