Duplicating historical black-and-white negatives is a principal method of preserving them. It applies in particular to negatives made on inherently unstable plastic supports, such as cellulose nitrate and early cellulose diacetate films. This tutorial paper presents a review of the pertinent 20th century literature and describes a general procedure, which is based on the application of sensitometric principles. The resulting duplicate negatives are evaluated in terms of the quality of prints made from them as compared to those made from the original negatives. -- AATA