I had this.
I went to sys prefs and unchecked calendar. Selected 'delete from my mac' wait a few seconds, then re-check calendars and select 'merge'.
If you want to try this I would suggest backing up your data! As I said this worked for me but you may find different.
I did a similar thing from my iPhone, since that's where the duplicate entries were. I repeat the suggestion to confirm that your data is backed up, on your Mac, for instance, before doing this:
- on the iPhone, go to Settings
- toggle "calendars" OFF
- at the prompt, select "delete from my iPhone"
- at this point ALL your calendar data should be deleted from iPhone
- reboot iPhone
- go back to Settings, toggle calendars ON
- after a few minutes, all the data is back, but without duplicates
I surmise that the original sync during the iCloud activation had a bug or redundancy.