MS Office 2021 - Receive every Email twice

[BWalter]

Hello colleagues, I know, a lot has been written about this topic, but nothing has helped me so far. The following: I used Office 2021 on one PC (win11) and on two laptops (win10+11). At T-Online I have two email addresses and both configured via pop3 in Outlook. It has been working fine for a long time.

Now I have also installed Office 2021 on a Dell Latitude 5285 with Win10. All now 4 computers with Office 2021 are licensed and have exactly the same settings! Only on the Dell all emails appear twice, no matter which email was addressed. As a test, I took an email address out of the account, then emails for the remaining address only appear once. But I don't want it to be like that and it works perfectly with the other 3 computers. What can be the reason for this?

I don't know if this has anything to do with it, but it's unusual and only on the Dell Latitude. Under Send-Receive Groups in the edit window "Delivery Settings-All Accounts", the two buttons "OK" and "Cancel" are missing in the bottom right. So I can't make any changes here.
I would be happy about every super tip!