The appearance of doubling can be caused by several different things. Machine/mechanical doubling is not considered an error and it can be quite common on some issues. Die wear can also create a doubled look. A true doubled die will have split serifs- the overlapping letters will be notched and separated. That notching is an important diagnostic. The differences are hard to describe and looking at examples is the best way to learn. Check out the link for descriptions and pics of the different kinds of doubling.
http://koinpro.tripod.com/Articles/...Doubling.htm
http://koinpro.tripod.com/Articles/...Doubling.htm


















