Same event in multiple calendars

Is your feature request related to a problem? Please describe.

I recently migrated from Google calendar and I am used to this setup:
I have imported my girlfirend's web calendar and then I have my personal calendar. My events are in the personal calendar and I have her calendar to easilly know when she is available when planning events (I usually have it hidden).
When we plan an event that is for both of us we send an invite to each other. But here is the problem. When I get the invite and Nextcloud has already synchronized girlfriend's calendar, I can't add the event from invitation to my personal calendar. It ends up with a general 'Failed...' error in the gui. It seems that Nexcloud treats the event id as globaly unique? Because when i accept the invitation before girlfriend's calendar synces it works ok. Or maybe the problem could be that it tries to update the event that is in the remote read-only calendar.

Describe the solution you'd like

I think that event id should be unique only within a calendar to be able to add the copy of the event from imported calendars into my personal calendar. Also nextcloud should not try to update imported read-only events.

Describe alternatives you've considered

I can remove girlfriend's calendar, but that doesn't solve my needs.
I can create new event in my calendar, but that breaks event updates and event responds.

Additional context

No response

Reproduce the sequence in the GUI: synchronize the imported read-only calendar, then accept an invitation to the personal calendar. Trace invitation acceptance and calendar synchronization around event IDs and read-only events; done means the invitation creates a personal-calendar copy without failing or attempting to modify the imported event.