In California they no longer verify your residence or validity of voter's registration with mailings. My mom remained on the rolls of a precinct she hadn't lived in for over 15 yrs! People should be required to re-register periodically or prove who they are when they vote. Registrations should be purged when there is no activity, but instead, I fear they are exploited!
Voter rolls are public. A little investigation and research will let you uncover duplicate registrations and get them cancelled... but you need to start now.