Create failed for User Computername\Username

Hello,

After creating a new SSMSExpress Login username account, Iuse it as the Database User of the attached database (aspnetdb.mdf), but Ireceive this error.

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

->The login already has an account under a different user name. (Microsoft SQL Server,
Error: 15063)
...

I am sure such username account is not yet members of that database(aspnetdb.mdf) for this are the users present
-dbo
-guest
-INFORMATION_SCHEMA
-sys
-COMPUTERNAME\ASPNET
-CONPUTERNAME\IUSR_COMPUTERNAME

cheers,
imperialx

Logins and users are two different things.

Logins are defined at global level. Users are simply a mapping from a login to a username.

Try opening SSMSE and open the security folder. Under logins you will see all the "logins" (users) that have already been created. I bet the user you are trying to create is in that list.

Hope it points you in the right direction