People question why this happens on the mintmark and not the date as well, where at other times it will...why is this?
Notice the depth of the details, the mintmark has been formed deeper into the die, hence will be the first point to sense movement of the die in the squeeze method of minting. It then slides down to make the full impressions of the less deep elements...
It is not a RPM because that ended in 1989. Starting on 1990 forwards the mint marks are part of the design from all the mints. Philly only had a 'P' mint mark on the 2017 cents. Machine Doubling the machine caused the doubling after the stike. Common. Doubled dies have the doubling on the die. Not caused by the machine. Doubled dies can have MD, but the die doubling is what makes the value for them. Machine Doubling is actually damage to the coin after the strike. But if you have coins that have Machine Doubling on them and have them graded, they don't consider it a negative. But to a prospective buyer they are a deal breaker for me.