macOS Calendar adding its own alerts
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.