1974 50 Cent Piece Error

The issue with your coin is called machine damage/doubling. This is not the same as hub doubling/doubled die. In the case with your coin, the die is normal but because of die movment, it damages the devices with slight movement during the strike. This happens a lot and to me it adds no value to your coin. (because I view the coin as damaged) This happens a lot on the half dollars and Ike dollars. (the larger the coin it seems, the more chance of it happening) The coins that are collectable to variety collectors is where the die during the hubbing process, gets doubled. (thus the term doubled die) What is the difference? The dies themselves is the difference.

(MD is from a normal die and doubled die is from the die being hubbed incorrectly) On MD coins they can vary from strike to strike, but on a doubled die the devices a pretty much the same from strike to strike. (making allowances for die wear) So the basic rule;
If the raised devices are reduced in size, then it is MD.
If the raised devices are enlarged in size, then it is double die.
Incuse devices have the opposite affect.

Note in the example the devices on the left are reduced in size compared to the center normal one. Then note the image on the right how the device is enlarged compared with the center image. Hope this helps. If not ask another question.