I worked the polls in Wisconsin. I know how the system works. Every state should use this system:

Feel free to flood the streets with ballots. Drive through neighborhoods and dump them out the windows. Mail stacks of them to every address in the state.

If you're not registered you're not getting your vote counted. And nobody's casting a ballot twice. It's literally impossible.

All the mail in ballots are delivered to the locations where the voter would cast their ballot in person. A polling official cosplays at the voter and walks through the process as if the vote was being cast in person. A second ballot cannot be cast by mail or in person. If a second ballot is attempted to be cast under the same registered voter it is immediately noticed and opens an investigation.

The only thing you could do is try to cast a vote in the name of another registered voter hoping they don't vote in that election and that they don't notice a ballot was cast in their name so the fraud goes unreported.

But that's why it's important we get ballots into the hands of every registered voter and ensure everyone votes in every election.