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.

How to enable SQL Server Agent?

How to enable SQL Server Agent

Procedure:

Execute the following Queries and refresh the server

Step1:

exec sp_configure 'agent XPs'
if it display an error that advanced options are disabled then by running the following query enable the show advanced options and reconfigure

Step2:  Run the Query to enable advanced options

exec sp_configure 'show advanced options' , 1
go
reconfigure
go

step 3:
exec sp_configure 'show advanced options'
go

Step4:
Enable agent XPs by using the query

exec sp_configure 'agent XPs'
go
exec sp_configure 'agent XPs',1
go
reconfigure
go
exec sp_configure 'agent XPs'

Step5:
now restart the SQL Server Agent / Server. so that SQL Server Services should be enabled

still if it doesn't start then

Step 6:
goto start-->run type services.msc and press OK

Step 7:
Services window will be opened then goto SQL Server Agent, Right click on it and go for start

Now we can see the SQL Server Agent started in our Object Explorer