Remove Duplicate Entries on "My Calendar" Under Google Calendar App?

As Apple surely didn't make it easy to move my data to my new phone, I had to re-start the process several times, such that when I open the "Google Calendar" app, I see duplicate entries under "My Calendar" -- see screenshot below.

I would like to simply remove all of that. However, when I access Google Calendar through the website, I don't see all those duplicate entries. They seem to exist only on my phone.

I do not see those entries using the built-in "Calendar" app.

If I enable "My calendars" I also see the duplicates under the "Calendar" app.

How do I delete "My Calendar" under Google Calendar, along with all its content?

Thanks in advance!