"The Cleanest Election in History". Sure.

And the cow jumped over the moon.

We estimate 3% of the U.S. voter population is registered to vote in two states. Which state these double-registrants choose to vote in reflects incentives and costs, being more prevalent in swing states (higher incentive) and states which automatically send out mail-in ballots (lower cost). We call this behavior cross-state strategic voting (CSSV) and estimate there were 317,000 such votes in the 2020 presidential election.  --National Bureau of Economic Research quoted at PowerLine

Were they all double-votes?  Nope.  Likely were 60,000 doubles.  That's enough to have 'swung' at least two battlegrounds.

Note well:  Wisconsin not only sent out mail-in ballots, but also was a 'battleground' State.  Democrats outnumber Republicans in the 'two-State registration' space.

Do the math.