Machine Doubling happens after the strike, damages the coin most of the time in the same direction on all /some / or hit and miss. Not a doubled die, as a doubled die is formed on the die, thus the term doubled die.
Machine Doubling can happen the same way on a doubled die as well. The MD doubled dies, are one not preferred to a variety collector. (I advoid them like the plague) Pass on them it MD is present on a coin I'm thinking about buying. If a coin is graded, the
Machine Doubling doesn't affect the grade. But I would never buy a doubled die with MD on them.