Credentials are a username/password combination. In this case, you already have a logon session started with a set of credentials and have tried to start another session between the same computers, using the same set of credentials.
Either end the existing session, or log on with a different set of credentials. Note that this message may not be caused by anything you did at the user interface, but can be caused by a running application (applications can log on as users). In that case, contact the supplier of the running application.