SQL Server Error 15023: User already exists in current database
This error occurs when users in a database are orphaned. Orphaned users are those users for which there is no login id associated with the database user. This is a common occurence after restoring a database from a backup from one server to another. Here’s the fix – run this query within the affected database.
EXEC sp_change_users_login ‘Update_One’, ‘<database user>’,‘<SQL Server Login NAME>’