Trump is right. If the system works as intended, someone who goes to vote in person, who has already voted by mail, will be flagged as already have voted. If they cast their ballot in person and then their ballot shows up by mail, it too will show up as they have already been counted.

Democrats know the system is broken and want to be the only ones who that favors. We can legally (as far as I know) do both. Maybe this will force them to address election integrity (but I doubt it will).