Machine Doubling is caused by the machine.

Note the alteration of the devices from a normal die on this coin. It was sent into the graders at PCGS as a doubled die, but only got slabbed as a normal cent coin.
Over 100 coins were slabbed a normal coins, when the owner thought it was a doubled die:

A doubled die coin comes for a die that was incorrectly made that has a doubled hub issue.


Note the devices are larger than normal devices and doubled looking because of a hub issue during die creation.


Being that
Machine Doubling can happen on normal and doubled dies, because it is a machine issue.

Note the yellow arrows on these images. Those are the areas affected by the post strike
Machine Doubling. On the real doubled die, the date will can show hub doubling, but the mintmark will not show mintmark doubling as they dies and mintmarks were two different processes. On this die, the mintmarks should be normal. Thus you see the same alteration on the date and the mintmarks of
Machine Doubling in the same direction. But on the dates you see enlargement of the devices. So these are examples of
Machine Doubling and a doubled die. But most of the time they are not as often to happen on the same coin. (But if the machine was the issue, then it will happen)
But only a doubled die, creates a doubled die coin. (With or without
Machine Doubling added to the coin post strike)

Here is a closer view of the doubling:


IS having a doubled die with
Machine Doubling is a good thing. Well to new collectors they will buy them, but to a seasoned collector, they will pass on these. Why have something confusing on something you want to show to others. So to me they are a problem coin and not desirable. Just give me the normal strike in an earlier die state of a doubled die and that would be my happy day.
But
Machine Doubling is never a doubled die. It is a post strike striking event that alters the coin

Note the affected area is showing flat altered machine push. Also note the devices have a 90 degree on angle on the pushed areas.



The devices are always normal sized, but reduced with the
Machine Doubling. Thus they are smaller than normal. On the newer die processes single squeeze dies, they are hub pressed once, but the movement can be slightly off and move during the push snapping into the correct locations.
Huh? This alters the central areas of the design. make/altering the devices slightly.

Note they affected areas are to the right or left of the statue on these doubled die with a small/med/large bar showing.
These can vary in size of width and lengths, but all come from different dies:
https://imgs.inkfrog.com/pix/coop49...ar_Cents.jpg
Sometimes the angle is not correct and is twisted a bit:




When the hub process is complete, it alters the un-formed areas in the center of the design. With the different designs being affected differently. So you need to look these years up to see what is a doubled die on the obverse and reverse could look like. (Otherwise you will be finding
Machine Doubling or extreme die wear on the coins you search through) This is pretty much a small measure of what there is on this subject. But this is the differences:
1.
Machine Doubling is
caused by the machine. It can vary from strike to strike with the same dies. The coins are victims of post strike damage on the coin on the dies rise after the strike.
2.
Machine Doubling can happen on doubled dies just as it does post strike alter the normal and doubled die coins:
3.
Machine Doubling is never a doubled die. It is a striking event. Happens a lot, not rare.
4.Doubled dies are
from a die that has the design doubled.
5.
The reason these are desired is because they are worth more than face value.
Machine Doubling is just a face value coin. On a doubled die, they are just a distraction but does have value. (Just not as desirable to a seasoned collector) Other die events are not a plus most of the time. But another time for that question to be covered later when asked.
CoopHome:
Why Does Everyone Think They Have A Doubled Die? question asked on/and answered on this thread.