After years of using Ninox we still cannot solve this problem of the calendar duplicating events on our apple calendars.
We have synchronized the Ninox calendar with our iCloud calendar. When we enter a new event it invariably gets duplicated. Sometimes (a few times) even 10-15 times or 30-60-100 times. We have never been able to find a solution: we have searched the community, outside, asked support. Nothing.
I try to write this request hoping that in the meantime, as the years go by, someone has found a solution. Thank you!