1) This is the best Solution.
First of all run following T-SQL Query in Query Analyzer. This will return all the existing users in database in result pan.
USE YourDB
GO
EXEC sp_change_users_login 'Report'
GO

Run following T-SQL Query in Query Analyzer to associate login with the username. ‘Auto_Fix’ attribute will create the user in SQL Server instance if it does not exist. In following example ‘ColdFusion’ is UserName, ‘cf’ is Password. Auto-Fix links a user entry in the sysusers table in the current database to a login of the same name in sysxlogins.
USE YourDB
GO
EXEC sp_change_users_login 'Auto_Fix', 'ColdFusion', NULL, 'cf'
GO

Run following T-SQL Query in Query Analyzer to associate login with the username. ‘Update_One’ links the specified user in the current database to login. login must already exist. user and login must be specified. password must be NULL or not specified
USE YourDB
GO
EXEC sp_change_users_login 'update_one', 'ColdFusion', 'ColdFusion'
GO

2) If login account has permission to drop other users, run following T-SQL in Query Analyzer. This will drop the user.
USE YourDB
GO
EXEC sp_dropuser 'ColdFusion'
GO

Create the same user again in the database without any error.

Original Link

http://blog.sqlauthority.com/2007/02/15/sql-server-fix-error-15023-user-already-exists-in-current-database/

Pinal Dave