  Building a repository of proof-checked mathematical knowledge is without any
doubt a lot of work, and besides the actual formalization process there also is
the task of maintaining the repository. Thus it seems obvious to keep a
repsoitory as small as possible, in particular each piece of mathematical
knowledge should be formalized only once. In this paper, however, we claim that
it might be reasonable or even necessary to duplicate knowledge in a
mathematical repository. We analyze different situations and reasons for doing
so and provide a number of examples supporting our thesis.
