Hi Jason, It most certainly could have mechanical doubling (strike doubling) on it in combination with DDD, but after seeing hundreds of DDD nickels in my time it was an easy determination for me to say that it was indeed DDD. The obvious factors are the "double" letters ALL AROUND the initial lettering, not on just one side or the other. Take care!
That is the way I saw the images as well. Doubling on all sides of the letters. They must have banged the heck out of 1999 Jefferson dies eh? I have seen a lot of these for this date ... have you? Jason