The login already has an account under a different user name

Question

When I execute this SQL:

USE ASPState
GO
IF NOT EXISTS(SELECT * FROM sys.sysusers WHERE NAME = 'R2Server\AAOUser')
CREATE USER [R2Server\AAOUser] FOR LOGIN [R2Server\AAOUser];
GO

I get the following error:

The login already has an account under a different user name.

How do I know what this different user name is for my login account?