"Error 15023: User or role '%s' already exists in the current database."

Got this error recently when I downloaded a clients database from a shared host to my local machine. Tried to add the same user.

This article resolved it for me: http://www.codeproject.com/database/OrphanedSQLUsers.asp.

Here is the SQl that I ran "sp_change_users_login 'auto_fix', 'user'"