Sorry, but ballots are not identified as to who cast them.
True, and therein lies the problem. At what point are there enough illegal votes to invalidate the election?
I work elections in a state with open primaries. In our recent runoff, anyone trying to vote in the Republican runoff who had voted in the Democrat’s primary was promptly sent to the other side of the room.
It is unfathomable to me that these votes were allowed by election personnel — unless there was corruption involved. I don’t know enough about the demographics of the affected precincts to guess.