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 git@github.com
# Connect to GitHub.com 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 account on GitHub.com 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.
To force SSH to use only the key provided on the command line, use -o to add the IdentitiesOnly=yes option:
ssh -v -o "IdentitiesOnly=yes" -i ~/.ssh/id_rsa git@github.com
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.