Germany uses 100% pen-and-paper ballots, so cyberattacks cannot target the ballots directly, FWIW. Mail-in ballots? There’s probably always been certain numbers of “assisted voting” cases, especially with the elderly.
Voters are registered in their district of residence, and lists are kept at the polling stations, where you have to provide ID. You’re then struck from the list. Don’t see how multiple votes by one person would be possible.
Russia?