So, you changed a SQL server database to a new server and now you can’t add the old user for this database.
And after every single attempt to add the user name you see this error message: “Error 15023: User already exists in current database” That means your database has an orphan username.
In order to solve this open a SQL Query Analyzer session and connect to your database (with the ‘sa’ username) and then run this query:
EXEC sp_dropuser 'orphan_username'
This query will do the dirty job and drop the user from the database, now you can add the user with this query:
EXEC sp_adduser 'orphan_username'
Or through the GUI, easy, don’t you think?