I checked what Settings/Prefs I could find. Settings have gotten bloated on Apple devices. I opened the two app on my phone, Mac and iCloud account, and checked several entries on both apps. In the notes I added which device had the correct device, until most of the entries were synced and correct.?

But not all, so I called Apple Support. The rep had me check the various settings, for both , then noticed that the iPhone had a message about 14 duplicate entries. I said that I would fix those after the call, as finally they all synced.?

Well, most of those duplicates were very old. They got created when upgrading OSes or upgrading new devices. Some were not really duplicates but when moving the files, the phone numbers got rotated one number. The top number became the bottom one if there were multiple numbers. Or one was a company entry and the others were individuals in different departments or even different locations. Some of the duplicates had previously been manually combined and the extras deleted, but I still needed to hit the merge entries button to clear then now.?

It has only been a couple of days and no new entries on either app, so I can’t say the issue is resolved, but I hope so.

Calendar and Contacts are highly linked, but why duplicates in one would affect syncing on the other makes no sense, at least to a human.