Outlook / CalDAV Sync. constantly copies calendar entries

Dear Community,

I have to ask a rather stupid / easy question maybe also I am not completly right here. Nevertheless I hope you can help.
The nextcloud calendar app (web version) had a few duplicates. I expect a user setting provoked it. I deleted them in the web version and they are gone. Unfortuanelty the outlook clients (with calDAV Syncroniser) do have a multiple amount of copies (80+). Deleting in outlook does not help either. Once syncronising the entries come back.
I do not know from where.

Can you help here?


Nextcloud version: 27.1.3
Operating system and version: Ubuntu 22.04.3
nginx version : 1.18.0
PHP version : 7.4

The issue you are facing:
Multiple copies of calendar entries which come from nowhere and it is not possible to delete them.

Is this the first time you’ve seen this error?: Y

Steps to replicate it: I have no idea.