Help Android Pre-Installed Calendar: Mysterious Self-Duplicating Appointments

I have a very strange issue with the Android, pre-installed calendar. This is the calendar I used primarily, that is until it suddenly developed the issue of when I create a new appointment and save it, it suddenly populates that day with tons of duplicates of that appointment.

And when I set any type of notification, of course, each appears at once, making my phone useless until I spend tons of time deleting each duplicated appointment! Right now, I have disabled the damn' thing, but I really like the functionality of the app. Does anybody have a fix here? I chatted with Verizon, and the suggestions they made, making sure the appropriate sync features are checked under my Google account, but it still does the same thing! Pls advise.