I have managed to get 48 copies of a particular calendar on my iMac that I subscribe to (US Holidays but I don't think that is important). I know it is related to iCloud but even when I laboriously delete them from my iMac, they come back! When I log into iCloud.com the calendars there are just one of each calendar I subscribe to, which is fine.
All the calendars on my iMac are in the 'iCloud' section, rather than 'on my Mac'. Any ideas on how I get rid of all the excess copies and stop it happening again?
I've just checked my iPad and yes they are all on there too.
If I switch off Calendars in iCloud all my calendars disappear, when I switch them back on they all re-appear, so it's in iCloud that the duplication has happened. I'm sure it was when I was trying to set up iCloud Drive. Transferring data took ages and never seemed to finish so I tried several times and I guess I was duplicating what was copied each time. Not sure how I move a calendar to On My Mac, presumably once I had done this, I would need to clear the data off iCloud, although again I'm not sure how to. Sounds like a phone call Mick! btw I'm out from 10am until about 4pm on Friday, but we can leave it until next week if that is better for you.
How about logging in to the calendar section of iCloud (www.icloud.com using your Mac, not a mobile device) to see if they are all there? If they are, delete all but one.