Die Deterioration Doubling. the die is getting worn down and losing it's crisp, detailed edges, and shows a shelf like pattern of the letter next to (extension of) the letter itself. Versus true Doubled Die. see this ==> https://www.cointalk.com/threads/got-some-doubled-dies-on-these-nickels.325359/#post-3213433
face value MD/DDD is very common just from the usage of the dies to mint coinage. Early "state" of the dies should have no MD/DDD, but as they are used more and more (later "state" of dies) then the MD/DDD starts creeping in from their use to over use.