There must be a database set up so each person can only caste one ballot. There is nothing that prevents a person from using his address to caste a ballot in his/her precinct and using his friend's address to cast a ballot in another precinct. Example: I live in Auburn, Washington. I use my Auburn address to register to vote in Auburn. I caste a ballot in the Auburn precinct. I have a friend in Kent, Washington. I register to vote in Kent using my friend's address. I go to Kent to cast a ballot in the Kent precinct. If I have many friends that live in other cities, I can repeat the fraud literally undetected. Theoretically, I could caste many ballots for one political party.