Registering the Same Instance Multiple Times

I'm trying to register a SQL Server instance through the new SQL Server registration option in Enterprise Manager. I receive a message that says, A server with this name already exists, but the server doesn't exist in the SQL Server group that I'm trying to add the instance to. Why am I getting this message?

You'll receive this message if the server has already been registered through Enterprise Manager and added to a different top-level SQL Server group. You need to expand the other SQL Server groups in Enterprise Manager to see whether the server is registered in a different group.

Registering the same physical instance of SQL Server more than once might be convenient sometimes. You might want to connect to the same SQL Server instance by using different Net-Libraries for testing purposes or register the same server more than once under different user credentials so that you can test permissions. To register an instance multiple times, you create an alternative client alias that references the SQL Server instance. For example, you can register a SQL Server called MySQLServer more than once in Enterprise Manager by creating a client alias named MySQLServerNew and registering that name in Enterprise Manager. You create the alias by using the Client Network Utility. For more information about creating a client alias, read the SQL Server Books Online (BOL) topic "Managing Clients."