I am not very familiar with these but it dosen't look like machine doubling to me as only the D seems to show it... possibly this is a re-punched mint mark?? I'm sure someone who is more familiar will chime in shortly, cool find!
There more that 1 listed for 1964-D in Cherry Pickers Guide newest V-4.but copying it against the law & cointalk rules.but this link will help. http://varietyvista.com/ http://minterrornews.com/
This mechanical/machine doubling, yes, only one element of a coin This mechanical/machine doubling, yes, only one element of a coin even just the "D" can have this type doubling but I bet I could find other areas on this coin to prove the shift.
Rock I am not going to fuss but just read the rules about posting a copyrighted photo's.I sure Speedy can help. THE END!