"On Reduplication" is a study of the logical properties of reduplicative propositions, that is, of propositions having qualifications, like 'Christ "qua" God is a creature' and 'being "qua" being is the subject of metaphysics'. The focus is on what ways qualifications change the truth value and the inference patterns of simple, categorical propositions. The central class of reduplications is that in which the qualifications are introduced by a qua connective like '"qua"', 'insofar as', 'under the concept of', or 'in virtue of the fact that'. Reduplicative propositions occur frequently and importantly in both traditional and contemporary philosophical works, but there has been little modern analysis of them. This study presents, compares and analyzes the different theories of reduplication that have arisen in Western philosophy. Texts are presented and explicated, and their significance is weighed relative to modern logical theory. Throughout this study, some important applications of theories of reduplication are noted, such as Leibniz's qualification of the principle of the Identity of Indiscernibles, Ockham's reduction of abstract entities, and Aquinas's view on the Incarnation. ! Uwagi: Obwoluta lekko zarysowana i wytarta. Brzegi stron nieco zakurzone i przybrudzone.