Just learned about D/D on coins this is what you see is mechanical doubling I always thought the same thing. Because you can see two but the difference is M/D is the one on top strikes a little bit over exposing the part on bottom so you see two. A D/D is where it puts the letter or number beside it so you see two complete ones side by side. Look at 1955 wheat penny that is D/D you dont need magnifier to tell.