This is a brand new problem for us. I do not think that this has ever happened before in Washington state. One thing to remember, there are cameras everywhere today and it is quite possible that the perpetrators are on camera.
At any rate people that voted can probably vote again. Though who one voted for is not tracked a the fact that one voted is tracked. Names and signatures are checked when the ballots are turned in. Once verified the vote is separated from the envelope. It is not looked at. They are stored separately. I voted last week and since for the first time in a long time I did it from my home mailbox i was a bit concerned. So two days later I checked the website and it confirmed that my vote had been received and counted.
At the Washington site that was attacked they know the last time that votes were picked up there. The state is allowing those people to vote again. This does not allow unregistered voters to vote, they will already have to be registered to vote a second time. Nor does it allow double voting. As I said, they check and verify each ballot. and make a note of who voted or not. If someone tries to vote twice they will be caught.
State and federal authorities are investigating a deliberately set fire that destroyed ballots inside a drop box in southwest Washington.
washingtonstatestandard.com