How to get the userID from a custom ASP.Net Membership Provider w/ non-unique usernames?

I've created a custom asp.net membership provider that authenticates against a legacy database. The problem I'm running into to is how to handle users that have same username.

I've repeatedly tried convincing my boss that a username should be unique but this has been to no avail.

Update:

It does seem to be fundamental flaw with the authentication system, but how do I convince my boss that he needs to update his Powerbuilder code? I've been trying for nearly 2 years now and he thinks a unique (username,password) is good enough. I also want to store the passwords in a unidirectio开发者_开发问答nal hash instead of plaintext.

Something has to be unique. The user id is a guid which is unique, I have email address as unique so you can use that.