A double check die seems like a great idea, but I don't have space on the top of my Dillion 550B progressive. I have had my Dillion since the 1980's, to date no double charges. I am however always worried and try to follow a procedure that will prevent a double charge. The greatest risk for a double charge comes from stoppages that occur due to periodic jams. Most of my jams are primer jams, or bullet canting in the seating die that ruins a case. Stopping to clear a jam, and then continuing, creates the highest probability of double charging a case, and someone with a progressive needs to think about always removing the case under the powder funnel and emptying it, before resuming operations.
My powder funnel throws a consistent charge so I am not worried about the throw weight changing by accident.