Procedures vary from state to state, but I think most states use a multi-partisan election board to screen incoming ballots. The board is typically composed of a mixture of Democrats, Republicans and independents. As absentee ballots arrive, they compare the signature on the envelope with the signature in the pollbook. As each ballot is accepted, the voter is “checked off” in the poll book, indicating that the voter cast a ballot. This prevents a voter from casting multiple ballots. It’s not absolutely 100 percent foolproof, but pretty close. This election may be plagued by Trumpite attempts to submit bad ballots to overload the system or create the perception of chaos. For example, I could mail in 10,000 empty envelopes just to create more work for poll workers. Of course, anybody pulling such a stunt is subject to federal charges of interfering with the mail, which will sic the postal inspectors on their ass. The Trump people are not just criminals, but reckless criminals, so they will probably attempt some tricks without regard to consequences. I know election officials out here on the left coast are just waiting for them.