Seems to me if the registered voter rolls are an on-line database, when they check in at one polling place, they are checked into the database as voting, which would then show up at every other polling place where they tried to vote again. Sort of an electronic version of the blue ink on the thumb.
Maybe not trivial to put in place, but certainly doable.
The 38 Early Voting Polls are connected electronically. The 850 precincts in our county on Election Day are not.
Good idea, though.
The biggest hole for fraud is VRCs for non-existent people. See my previous posts to see how these are created.