This error occurs when you try to add a key that's already been added to another account or repository.

Finding where the key has been used

To determine where the key has already been used, open a terminal and type the ssh command. Use the -i flag to provide the path to the key you want to check:

ssh -T -ai ~/.ssh/id_rsa
# Connect to GitHub using a specific ssh key
Hi username! You've successfully authenticated, but GitHub does not
provide shell access.

The username in the response is the GitHub account that the key is currently attached to. If the response looks something like "username/repo", the key has been attached to a repository as a deploy key.

Fixing the issue

To resolve the issue, first remove the key from the other account or repository and then add it to your account.

If you don't have permissions to transfer the key, and can't contact a user who does, remove the keypair and generate a brand new one.

Deploy keys

Once a key has been attached to one repository as a deploy key, it cannot be used on another repository. If you're running into this error while setting up deploy keys, see "Managing deploy keys" in the GitHub Developer documentation.