A bit strange, but in dealing with the recent family account jazz, I think it would be a pretty good idea. The basic concept is when you're registering you can have a box or something that says " Do you have another family member already registered on the site? " if checked, then another question appears saying " Who? - enter name of family member - "
This way the rules that they should be obligated to go by can actually be completely enforced, making it to where they won't get banned for duplicate IPs, but can't up-vote each other or answer each other's questions.
And maybe making a way for it to automatically do this if an IP that already has an account registered tries to register another account. That would prevent duplicate account issues pretty well. Just some suggestions.