Over 2 years and 3 major OS revs. but I finally worked around some of the local Calendar syncing mass-duplication issue.
The problem seems to be caused by repeating events that are missing the Time Zone field. This can happen if “All Day” is set for the event, or if “Settings / Advanced / Turn on time zone support” is NOT checked. I don’t know if, missing that entry, the parsing code is out of sync and plugging fields into the wrong variables?
It took some work to edit the suspect events in my calendar, but my desktop Calendar seems to be correct now.
This alone has made Calendars much more usable under Sequoia and iOS.
The catch is… initially the problem was when I synced w/ Ventura or later, repeating events (monthly, etc.) would become everyday repeating events. The result was a Calendar so cluttered as to be unusable.
The problem now is that now when I sync to my iOS devices, the events I edited to correct the issue on the desktop are synced, but also some of the old versions of them are synced to the iOS devices. The desktop remains duplicate free. I did not duplicate any of the edited entries, so there shouldn’t have been anything like that remaining. But there is.
First of all, the “Replace Calendars” checkbox claims to completely overwrite the iOS Calendar, but it does not “appear” to. At first, I thought that the older events must be lurking on the iOS device, but after first trying to completely delete the target Calendar, and finally, actually doing a factory reset, the old events still appear after syncing (again, a “Replace Calendars” sync).
The errors are clearly still coming from the desktop. I saved off the current Calendar as an archive, and separate Named Calendar exports. I then deleted all the Calendar data I could find (~/Library/Calendars and ~/Library/Group Containers), opened the (empty) Calendar and imported the separate Calendar exports (1).
The erroneous events still were copied to the iOS device (but still are not showing up anywhere on the desktop.
This is as far as I’ve gotten. Does anyone have any idea where Apple is hiding the rest of this data?
I should be clear that the above has greatly improved the overall issue, just not as good as I would like.
(1) I used the separate Calendar exports rather than the unified archive, because I have found them more reliable in the past. I may try the archive yet.