I'm glad you like the plugin!
I think I understand what you are asking for, but there are a few flaws in the logic.
WP-Members runs off the WP users table, for which there is non-duplicatable primary key. Also, there is a process to prevent users from having duplicate user names AND email addresses.
However, there is no way you would successfully completely prevent a situation like you are speaking of(such as firstname.lastname@example.org re-registering as email@example.com) because there is no way to know the following:
- The user's primary key and username (if the user is not logged in, which they are not during registration, there is no way to know this)
- We can already prevent a duplicate email from signup (such as firstname.lastname@example.org already has an account)
But in the situation you are talking about, you have far too many variables to account for and therefore no way of doing "on-the-fly" prevention. For example, what about email@example.com vs. firstname.lastname@example.org. Here are two email addresses that could be related in the same way as the example you provided, yet they could be two completely different and unrelated people. Yet you have no way of knowing that.
It is for this exact reason that the plugin has the capability of moderating registrations (i.e. requiring admin approval before the user account has access). I developed this workflow on a newsletter site of my own where I had the same concerns and we did have people trying to signup to circumvent the process (there was a free trial involved and people were signing for additional free trials with a derivative email address). Since there was no good way to prevent this dynamically, we instituted a process of verifying our registrations before they were activated. You can do this with the plugin by using the setting "Moderate registration" as checked. Then a new user will need to be approved by an admin prior to having access.