Thanks for your reply. That is not the issue. Let me try to explain again.
The login "qc" has rights to 16 different databases. For 13 of the databases, when I run sp_helplogins, the login "qc" has the user_name "qc" into the database. For 3 of the databases, somehow the login "qc" has the user_name "db_owner". Those 3 databases are not accessible for the users via the application. I cannot find a way to change that relationship.
If I look in the database under security, the "qc" user does not exist in the database. If I run sp_adduser 'qc", I get the Msg 15063 error stating that the login already has an account under a different name -- which is dbo. I cannot run sp_dropuser as the 'qc' user does not exist. When I go out to login properties for the 'qc' login, I cannot uncheck the box for that database as when I try to save it, I get a "cannot drop user dbo" error.
I cannot find an answer. Any suggestions are welcome.