But even then, you still don't know if they are different people or not.
Nevertheless, I just thought of a realy useful mod/hack to stop help with this (for all you coding wizards out there), a module which checks the originating IP and forces subsequent sign-ups from the same IP to have their registration moderated (approved). Adding a time frame check to that would make it even more useful. So if from the same IP and within say five days, require approval.
You may get more attention posting in the Modifications Requests (unpaid) forum.