After restore a database with override option, while trying to map a user to that database it will throw error as : User, group, or role 'hello' already exists in the current database. (Microsoft SQL Server, Error: 15023)

TITLE: Microsoft SQL Server Management Studio
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Create failed for User 'hello'.  (Microsoft.SqlServer.Smo)

For help, click: http://go.microsoft.com/fwlink?ProdName=Microsoft+SQL+Server&ProdVer=10.50.1600.1+((KJ_RTM).100402-1539+)&EvtSrc=Microsoft.SqlServer.Management.Smo.ExceptionTemplates.FailedOperationExceptionText&EvtID=Create+User&LinkId=20476

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ADDITIONAL INFORMATION:

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

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User, group, or role 'hello' already exists in the current database. (Microsoft SQL Server, Error: 15023)

For help, click: http://go.microsoft.com/fwlink?ProdName=Microsoft+SQL+Server&ProdVer=10.50.1600&EvtSrc=MSSQLServer&EvtID=15023&LinkId=20476

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BUTTONS:

OK
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In this case we have to delete/drop that user from that particular database, again we have to create a user under that database.