I have successfully reproduced this behavior using both a GSuite and plain old Gmail/Google account. Any kind of explanation for this behavior would be greatly appreciated.
Steps to reproduce:
- In Google Calendar, click the 'gear' icon > Settings. Click 'Add Calendar' > 'Create New Calendar'. Fill out name and click 'Create calendar'. This will be your secondary calendar.
- Click any space on the calendar to create an event. Provide Title and Time.
- Add at least one guest.
- Where it says 'Google Calendar' (or, for GSuite accounts, where it says your name) click and change the calendar selection to be your secondary calendar.
- Click 'Save' and 'Send' to confirm sending the calendar invites.
The event should appear once on the day in question.
After a few moments, the event appears a second time.
- Under 'My calendars' if you uncheck the secondary calendar (or your primary calendar) it will toggle the two events' visibility.
- Under 'Settings and Sharing' for the secondary calendar, under 'Share with specific people', it'll see that your secondary calendar is, by default, shared with 'you' and that cannot be changed or removed. I can't quite wrap my head around it, but I have a suspicion that this is a "feature" that is also inadvertently causing the seemingly defective behavior.