If I had found this one, I would have let it go as probably being Machine Doubling.
Look here : http://www.varietyvista.com/12%20Ke...intmarks.htm and peruse the pics of the different known RPMs on the Kennedy series. You will note RPMs tend to have either notching, or else the MM underneath the top one has distinct, rounded top surfaces just like the one covering it (unless the top one is worn).
By rounded surfaces, I mean to look on your coin and see the "doubling" along the D's spine has a flat top to it. Humor me and pretend you were an tiny ant crawling down off the top D. Your feet would hit a flat step that was the "top" of the second D.
With true RPMs, that lower "step" would be rounded like the top one would be.
As the the part on the bottom of your D, it looks as if something hit the MM and made a dent.
OMM= Over-Mint-Mark. It would denote a D or S or vice versa. But the correct term for a mint mark over a mint mark that is from the same mint would be RPM. Re-Punched-Mint mark. But this one is not even that. It is just Coin contact or Machine Doubling. RPM are not possible after 1989. In 1990 the mint marks were no longer added to the die, the mint mark is part of the design since then.