NK3C webauthn on gitlab says key already registered

I have 2 nitrokey 3C NFC.
I set with pivy-tool a pin for my first nitrokey. (can not reset this pin already… but that’s another issue).
And i have registered the second as second factor on gitlab.com.
Trying to register the first also as 2FA-Key gitlab says, that this key is already registered with this site.

No… the other one is registered…

How can this happen?