- Quit (exit) the Calendar (and possibly the Mail) app.
- Remove your Google account from the "Internet Accounts" section in "System Preferences".
- Remove the "Calendars" folder from "Library" by running
sudo mv ~/Library/Calendars ~/.Trash
. - Remove the Calendar caches by running
sudo mv ~/Library/Caches/com.apple.iCal* ~/.Trash
. - Remove the Calendar preferences by running
sudo mv ~/Library/Preferences/com.apple.iCal* ~/.Trash
. - RESTART the computer. This is a must. Otherwise, it does not work.
- DO NOT ADD your Google account back from the "Internet Accounts" section in "System Preferences". Instead:
- Open the calendar app.
- Go to preferences (CMD-,).
- Click on "Accounts".
- Add your Google account.
Now, it should work.
https://eshop.macsales.com/blog/38684-tech-tip-fixing-calendar-duplicates/