Images that are clear and sharp would help. Extreme close up (30x) on the part of the coin in question. You thread title mentions double on the D, but does not specify which. Is it mint mark or motto, each found on obverse, or is it the last letter of "United", found on the reverse?
Where the "doubling" occurs, and the specific characteristics of the "doubling" will be very telling as to what type of doubling has occurred. The most common is "strike doubling", also known as "machine doubling", and several other terms. Most cherrypickers guides touch on this very well.
Another possibility, if this coin is from the Denver Mint, is that of a repunched mint mark.
Yes, better pics are needed to make a fair assessment! If you are referring to the Mint mark, then it appears that there is spotting Northwest of the "D" Mint mark but it is not another Mint mark.