(from the point of the vote counters) They know because there are individualized bar codes on every envelope. They will just count every ballot that arrives intact. If they scan one and it’s from a voter who has already been counted, they’ll have to figure out if that’s voting fraud or a legitimate consequence of an act of arson/other problem–and it’s almost never voting fraud.

(from the point of view of the voter) assume your ballot was destroyed, get another one, vote again.