1963 & 1964 D quarters had lots of machine doubling on the mint marks it's very common. It would seem the mint mark was punched a little deeper in the die and that is why you see it on the mint mark and don't always see it on the other devises.
Wouldn't machine doubling show up on other characters? Looking closer, I see doubling both N and S, perhaps worn die spreading out. Agree NAV doubling.
Here is a link for the known 1964D RPM's. As you will see, there is not one that yours matches. http://varietyvista.com/09b WQ Vol 2/RPMs 1964-D.htm
"Wouldn't machine doubling show up on other characters?" When the mint mark is punched deeper in the die the mint mark on the coin is now the highest point. That is why you don't always see it on the other characters.
I always think when the "doubling" is on all sides of a device, it suggests die deterioration rather than machine doubling
One of the giveaways is having doubling on the top and the bottom of the mint mark. In my opinion, that would have had to a triple strike for that to happen.