Looks like die deterioration doubling or maybe environmental damage. The mintmarks at this time were hand punched onto the obverse dies. If a worker punched more than once, but the second punch wasn't directly on top of the first, this would cause an RPM (Repunched Mintmark). Now every coin made with that die will show the exact same doubled mintmark. The main reason I don't think yours is an RPM, is because the "second D" is larger than the mintmark. They should be the same size.
