It is not hard to find special collections librarians who believe that there are no duplicates, meaning that no two printed items made by hand are the same, even if from the same type, plate, or press.
This may seem funny to some since the very goal of publishing and printing is to make reproduceable copies of the same thing over and over again, but if you consider that all aspects of early books and printed matter were made by hand: the type, the ink, the paper, the binding, the illustration plates, everything, then differences between copies that were meant to be the same may be a little easier to understand.
Think of a batch of homemade cookies and how they all taste the same, but each is a little different, some are rounder than others, some with more chips, etc.
So when the American Printing History Association (APHA) during their conference "Learning to Print, Teaching to Print" came to visit the Special Collections Department of the Smithsonian Institution Libraries, there were plenty of things to show.
Consider these different copies of the same plate by Friedrich Heinrich von Kittlitz, a 19 century naturalist, artist, and explorer:
Each is printed from the same plate, but coloured by hand differently: some are spotted, some are not; some are striped, some are not, etc. This is not only interesting from a printer's and illustrator's point of view, but also from a scientist's point of view. In printing and the printing arts, there are so many variables that can influence the end product. This is why we say there are no duplicates and why, in part, special collections librarians and printing historians have jobs. We provide perspective about the historical and technical nuances of these handmade printed documents.
Other types of printed matter we displayed for APHA were modern handmade artist's books about the history of science, variant copies of an illustration in different editions of a Galileo work, an illustrated 18th century encyclopedia on how to print, a 19th century scientist's proof copy of printed illustrations with corrections alongside the original drawings.