Let’s say that you have a Dem who voted in both the Dem primary and the GOP runoff. That persons vote is invalid.
The question is, which candidate do you remove the vote from? Yes, one can assume that person voted for Cochrane, it’s actually flippin’ obvious. But legally, given the secret ballot, you can’t prove who that person voted for.
You could just ask them. But what if they refuse to say? What if they lie and say they voted for McDaniels?
You would invalidate the runoff vote. They weren’t supposed to vote in the runoff if they voted in the Dem primary.