If McDaniel could prove that, he would have strong grounds for an election contest.  Before the election, a joint Attorney General/SOS memo made clear that those who already voted in the Democratic primary could not vote again. But there’s no evidence offered so far that there are thousands of such voters or any of the other alleged “rampant irregularities” in the race.

If, on the other hand, McDaniel wants to go after Democrats who did not vote in the earlier primary on grounds they did not intend to vote for a Republican in the fall, that legal theory seems quite weak for reasons I’ve given earlier.