That's true, Tim, they do check before registering, for previous registrations of that number, and condition, but Indiana plates are not expensive even for a pair. I have two nice pairs that are in very good to excellent shape that I came from swap meets and even asset from an antique shop. Being "frugal", (read cheap), I doubt that I paid more than $20 per pair.
My problem with the state is that they require registering as an antique auto, they issue an antique plate that must stay in the car while you run on YOM plates. Antique registration allows only for occasional use limited to parades, club activities and "testing". I drive mine regularly and cannot be restricted.