Depends entirely how widely distributed they were.
There’s also tons of people on the voter rolls that never vote, are dead, aren’t living in the district anymore, and so on. Covering for ballot fraud is precisely why the Dems fight to stop the rolls getting cleaned up, and push the to prevent voter ID laws going in place.
As for doing it again. they clearly tried in places but with 350k poll watchers and lawyers everywhere, it was much more difficult. This sort of fraud is also not well coordinated, it’s more putting corrupt people in place and letting them do what comes natural. Arizona they are still trying it, that’s more coordinated and they feel more legally protected.