I recently got a MacBook, and I've added a few Google accounts to the Calendar app. Ever since I did, I've been getting alerts about the event after it's already passed or in progress. It seems as though Calendar.app is adding alerts to the event on its own; this didn't happen before I added my Google account to the macOS calendar.
The only alert that is supposed to be there is ‘Alert ten minutes before start’. Every other alert there has added somehow.
Best Answer
I've seen this behavior quite a bit with the calendar application when I dismiss the reminders. More specifically, if you snooze, "remind me tomorrow" or otherwise "kick it down the road", it seems to pick up these additional reminders. This is with a combination of iOS and macOS devices on the same shared calendar.
I think this comes about because you have a reminder several hours or days ahead of the actual event.
Suggestion? If you have something that you want to remind you on a daily basis, instead of kicking the reminder, just set it to a daily repeating reminder instead. That way you can get the benefit of being nagged about it but also you can just dismiss / mark completed today and hear about it again tomorrow without building up the multiple reminders.