Why are there duplicate contacts on my iCloud and how do I get rid of them?

I have seen several questions and answers around this topic but none of them really addresses my issue.

Since upgrading to Monterey on my iMac, my contacts contains duplicate cards as well as duplicate groups. (The duplicates also showed up on iCloud contacts, my iPad, and my iPhone.)

The discussion forum answers generally tell you to check out https://support.apple.com/en-us/HT203565, but that page tells you:

• If you see duplicates in Contacts on iCloud.com, visit Apple Support for help.

So if you go to that page and search for "duplicate contacts", you'll see intstructions you to how to delete/merge contacts but if you search for "duplicate groups", nothing relevant shows up.

So my question is, is there any way to delete duplicate groups in an automatic or semi-automatic way?

I have a lot of groups, and even after you select "delete group", you get a pop-up asking if you are sure, so it's really, tedious.

And why won't someone explain why this duplication problems happens with Contacts? Should I expect this to happen at every upgrade? Is there any way to avoid it in the future?