SQL Server 2012 Error 15023 : User already exists in current database.

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', 'LOGIN', NULL, 'PASSWORD'
GO


and you are set to continue your work leaving this problem behind.