Machine Doubling. On a doubled die you will see a spread on the centers of the devices. Here is three images. The center one is a rev that is normal with MD on the devices. The top and bottom images are DDRs. Note the width of the devices on the DDR's
When you look at the outside edges of the devices, that is where you will find the common Machine Doubling. But look at the centers of the coins and note when they are wider or taller. That is where to look. Not on the edges of the devices.