Found another error that we see a lot that I wanted to put check out there for you to know:
The error "another user is currently logged in" occurs when the attributes sent in the SAML assertion do not match the email address that was used to start the login process.
Check the attributes in the SAML assertion and make sure that they are an exact match to the ID the user is attempting to use, and also an exact match with the Admin Console. - How to perform a SAML Trace
Chances are the Email address is what is causing the problem or you have something funny with the user's name.