Most proof dies show an MDO, this is the second impression that you are seeing. Proof die have 2nd die pressings.
It's the proof coins that are struck twice (at least). https://www.usmint.gov/learn/production-process/coin-production Proof dies get special polishing, frosting, etc, but not a second pressing (at least not in the single-press era; previously all dies were hubbed at least twice). https://www.usmint.gov/learn/production-process/die-making Correct me with a source if I'm wrong, but that's the way I've always understood it.