Calendar event in Google Calendar.
If I modify the the calendar, to either iCloud or local calendar, it does not move the calendar, it makes a copy.
The result is: two calendar entries side-by-side.
This has been happening for a while (maybe a year or two).
This doesn't happen when I use Calendar.app to move events into Google Calendar.
(NOTE: Google Calendar app in iOS does not let you change the calendar of an existing event).