When you restore or migrating your MSSQL database from one to another server, below error message may appear:
Create failed for user ‘UserName’. (Microsoft.SqlServer.Smo)
An exception occurred while executing a Transact-SQL statement or batch.
User, group, or role ‘UserName’ already exists in the current database. (Microsoft SQL Server, Error: 15023)
You can simple to solve the issue by maps an existing user in a database to a MSSQL Server login: