GitHub "key already in use" - but I don't know where
In my GitHub I want to store my ssh pub key. But GitHub tells me: "Key is already in use". But I have never stored an SSH key for this account. I once gave my pub to a colleague so that he could store it in his GitHub account so that I could check out the project. I currently only have a pub key. I once heard that you can have and manage multiple pub SSH keys.
My two questions:
Can you only store one public SSH key on GitHub? Or can you store the same key per account?
Would the use of multiple pub keys help me here? And is this the common way / the right way?