Unable to add an SSH key


I’m trying to add an SSH key to my account.
I click on New SSH key button, write some title, paste the contents of my id_rsa.pub there, and click Add SSH key button. Then I’m redirected to Sign in to GitHub · GitHub , but it seems no key is added: I have “There are no SSH keys associated with your account.” in SSH keys field.
Can you help me fix it?

Update: I’ve been missing the error text all this time. It really doesn’t stand out. :-
So, it says " Key is already in use", I indeed have it in use in my other account. Can’t I share a key between two accounts?

The key is what’s used to identify you when you connect with SSH, so no.