https://www.realclearpolitics.com/articles/2020/04/24/28_mil...
https://www.whitehouse.gov/sites/whitehouse.gov/files/docs/p...
Coordination on that scale is completely impossible to pull off without detection. This just isn't a feasible attack.
There's a virtually zero chance that a group effort involving thousands of people isn't uncovered before the attack happens.
But that is getting ahead of ourselves. The problem isn't how do you solve duplicate voting, it is how do you detect it. That is a solved problem, already, and the answer is that there are very few attempts at voter fraud in a vote-by-mail system.
Now, the electronic machines are a whole 'nother matter. I would fight hard against my state implementing such a disaster. Vote by mail is great.