SQL Server 2008 user, group, or role already exists in the current database
Note : Whenever you move your database from one server to another
  server there is a problem of orphan user usually occur. FileListOnly
  is the new term in sql server that has all the details of the created
  backup that who has access to it.


So there are sequence of task that you have to follow


  
  Firstly you have to restore the FileListOnly from the created backup to the destination or new server.
  Restore the backup.
  Use the sp_change_users_login  as per the need. For help regarding this procedure you can refer
  http://msdn.microsoft.com/en-us/library/ms174378.aspx.
  


I put a example here hope it will help:

&gt; RESTORE FILELISTONLY FROM DISK = N'C:\YourDB.bak'
&gt; 
&gt; RESTORE DATABASE YourDb FROM DISK = N'C:\YourDB.bak' WITH MOVE
&gt; N'YourDB' TO N'D:\YourDB.mdf', MOVE N'YourDB_log' TO N'D:\YourDB.ldf',
&gt; REPLACE
&gt; 
&gt; exec YourDB.dbo.sp_change_users_login 'update_one', 'UserName','UserName'