SSH key problems. Key already in use. Authenticated even after removed

[jackra1n]

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So last week I started reading about git commit signing. First tested it out on my main PC, generated a GPG key, added to github etc. It worked. Cool.
Then later I found out you can use SSH key for commit signing. For me that seems better as preferably if I can use 1 key for both signign and authentication thats easier to keep track of and I dont have to install gnupg on my mac.
So I setup ssh signing and authentication on my mac. It worked
Then I wanted to also use SSh for commit signing instead of GPG on my PC. I try to add my SSH key as signing key but I get an error message "Key is already in use". Weird. I tried a few time but it doesn't work.
Today I decided to try it again. Same error message. Then I remove my ssh authentication key from my githubs account, try to add it as signing key, still the same error?
Then, I found Githubs page Troubleshooting SSH / Error: Key already in use, so I try it out and it returns a text saying that Im authenticated? Im not sure if I'm understanding all this correctly but I think this should not be the case if the key has been removed from my github account?

I dont know what to do now. I know I could probably generate new ssh key pair but preferably I would like to keep my current key as I use it for other authentications that might take some time to get changed to a new key.