I had looked at that article for the related problem of multiple calendars and did not find it very helpful as it advises contacting iCloud support when calenders are duplicated in iCloud.com as they are in my case. Phoned local (Sweden) Apple support. Totally useless. I did not read all linked articles, but will do that now.
Some article (can't find it now) further suggests using 3rd party SW such as Calender Cleaner to get rid of duplicate events, but the ratings for it on App Store makes me a bit wary. Any comments?