The ballots envelopes (not the ballots themselves) are keyed to the voter's identity. When the ballot is removed (not until the signature is verified and not co

The ballots envelopes (not the ballots themselves) are keyed to the voter's identity. When the ballot is removed (not until the signature is verified and not contested), the voter is counted as voted, so if they double vote, then the second vote will be rejected. Likewise if you try to vote by mail and then at the poll, you are flagged before you even try to vote.