Anyone interested in this coin: feel free to make an offer via private message. I have this coin and the D is completely above the shadow D. It's exactly as described by USA Coin Book, with the shadow D being very faint yet clear. This would be so easy to miss if I weren't specifically looking for it while grading & sorting all my 1956-D wheats.
I learned this: it seems the Denver mint had a really BAD year with their penny quality in 1956! I noticed many inconsistencies with the mint marks. There seem to be different sizes for the D and several various positions you will notice for the D. The "D over shadow D" seems to occur (and this is the case with my coin) when the smallest "D" mintmark is used and when it is tucked up high, next to the descending tail of the 9 in the the date.