Most mint errors discovered are minor and do not add much, if any, value to the coin; tiny die chips, planchet bubbles, abrasion (file) marks, die cracks and the like are not dramatic enough to add any value to a coin.
All mint errors are unique but if it’s an alteration of the die in some way and when more than one coin is found with the same difference it is usually adopted as a variety. It must be deemed significant and not common by the professional attributers of varieties.
Planchet division and Striking Division errors are unique and don’t result in multiple coins with the same error, but the Die Division can and this is where most varieties come from. If two or more coins are found with the same error (die alteration), whether on purpose or by accident, this often results in the coins being attributed as a variety.
Doubled Die is not the same as doubling, doubling is machine damage caused by a loose die and is not related to a doubled die since it falls within the Striking Division. Doubled Dies on the other hand fall into the Die Division and become varieties since two more more coins are usually known with the same die characteristics.