I was moving a lot of databases from one SQL Server to another, and my applications were giving me errors saying "Login failed for <user>". The user was already in the database with appropriate rights to allowed objects in the database. I tried mapping the user to the database and that's when I got this message: "User Already Exists in the Current Database"... I googled and found this very useful post about orphaned users when moving databases. These are the steps you should take to fix this...