Machine damage happens on regular die to coins during the striking process.
Hub doubled dies are created on the die it self during the die making process. Thus the term Doubled Die, because the doubling is on the die.
Machine damage is mostly flat, shelf like because of die movement during the strike. (Sideways movement of the dies and sometimes caused by die bounce. The doubling on a doubled die is usually seen as enlarged devices. Wider/taller than a normal coin set side by side in an image.

The larger the spread on the die between hubbings the more collectable the coin will be.



A lot of cents during the 1968-1972 years suffered MD on them:

Note the areas on the left side of the date digits.