Clearly A D Under The D Mintmark On This Penny? (1987)

The first pic I posted had me convinced there was a D under the D mintmark but I've also never heard of Zinc Rot. Thank you guys for clearing that up for me.

P.S. lol, that first pic had me going!

Well to start, cents in 1951 were made from copper rather than zinc with a copper plating.

(a) Solid bronze won't corrode in the same way as copper-plated-zinc. Without a plating, there's nothing for a bubble to form beneath. Also note on your coin, the white powder all around the mintmark, indicating corrosion is happening there. No such corrosion spot on the 1951.

(b) Know your series. Specifically in the case of repunched mintmark varieties, know when "repunched mintmarks" are technically possible, and when they aren't. Back in 1951, coin dies were still being sent out un-mintmarked, and the mintmark had to be added by hand.This created variabliity in mintmark location, as well as opportunity for errors/varieties in doubling of the mintmark. In 1989, they stopped doing this and added the mintmark as part fo the design of the master dies, so 1989 is the last possible year for a "repunched mintmark" variety. If you see a 1 cent coin dated 1990 or later and it kinda looks like a repunched mintmark, it 100% definitely isn't. Obviously in your case for your 1987 coin, this doesn't help, but it will help for future examples.

(c) Examine the coin in its entirety. There's a reason why we keep asking people to "post pictures of your entire coin, front and back, rather than just a closeup of the alleged error". That's because if a coin shows corrosion spots in other places, then it's highly likely that the entire coin has been subjected to a hostile environment causing corrosion, so any other weird "errors" manifesting on it are likely to be caused by that same corrosive environment. Your coin shows green discolouration all over it.