[masonic]

  everyone can only vote once

Does your registrar even check that? SV's doesn't even check if a given voter voted both precinct and absentee or if one voted in multiple precincts (e.g. prior and current neighborhood after a move to another precinct). They don't even check for existing registration in other precincts (let alone counties) unless the voter explicitly provides prior-address data.

[specialist]

Who is SV?

Every jurisdiction I'm aware of in the USA does ballot reconciliation, to detect double voting. Some are using electronic poll books, updated in near real time, to help prevent it.

[masonic]

Silicon Valley, e.g. Santa Clara County.

"Ballot reconciliation", executed perfectly, can only detect multiple ballots by the same registration.

I'm talking about an individual registered multiple times in separate precincts simultaneously.