So what happens is that you create a calendar entry, a record, which should get sent to GMAIL to handle. Then everytime you add a new device or SYNC, GMAIL tells that device it has a record available.
I'm guessing one of the devices may not be handling the sync back properly, which in turn...
That's just what I'm talking about, if the apps are creating / managing their own records, it can easily get out of sync.
Since you mentioned 365, do you use that as the mail provider? If so, have you tried removing them directly from the email host (outlook.com)