I know in Georgia if you move from one County to another there could be duplicate voter registrations. If you die or move out of state your voter registration may not be terminated. Either could allow duplicate votes. Another vector for mailed ballots is simply stealing them. If I know the state mailed them all two days ago and I received mine today, chances are my neighbors also got theirs. I could just steal their ballot, mark down who I want, and drop them all at a random blue mailbox.