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)

Additional Information:
An exception occurred while executing a Transact-SQL statement or batch.
(Microsoft.SqlServer.ConnectionInfo)
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:

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