ok i tested a bit further it works with firefox, so it should not be a general onlykey problem.
i found this bug: Registering with same username twice!!! · Issue #15 · duo-labs/webauthn.io · GitHub
which mentions that it might be bug/behavioural problem with chrome and their webauthn implementation (https://chromium-review.googlesource.com/c/chromium/src/+/1629587)
though i am not sure i completely understand this yet
the key i borrowed from a colleague was a yubikey 4, so it is not a fido2 key only u2f (afaics from the yubikey spec sheet) in contrast to the onlykey/yubikey 5. so my guess is that the fido2 implementation of chrome/chromium is at fault here.
our fix is to include the already registered devices in the excludeCredentials option on register, that
way, chromium shows the user that this key is already registered
thank you for your input 