The issue you are facing:
I have two calendars with the exact same name, but only one can have calendar entries, and only the other can have reminders/tasks. This problem only occurs with this one calendar.
Is this the first time you’ve seen this error? (Y/N): Yes
Steps to replicate it:
I don’t know how to do it, but it might be because of my iPhone.
My question
Is it possible to join seaminlgy duplicate calendars?
After extensive research, I’ve found workaround to merge two calendars into one.
Export all the calendars you want to merge;
Combine the .ics files following these steps (I did it manually, copy-pasted it into a new file, removing duplicate END:VCALENDAR and BEGIN:VCALENDAR);
Import this new file into NextCloud as a new calendar.