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Often the definition of a manifold includes a condition to exclude strange manifolds that are "very large" such as the Prüfer surface. One condition commonly assumed is that the topology on the manifold is second countable. This is necessary for embedding the manifold in Euclidean space for example. Second countability implies that there is a countable cover by coordinate charts, and in this case the proof of Poincaré duality in my book works without Zorn's Lemma, as stated at the end of step (3) of the proof on page 248. Perhaps I should have assumed second countability and skipped the part of the proof using Zorn's Lemma, but I thought it was kind of interesting that duality holds in the more general setting if one is willing to use Zorn's Lemma. I don't know whether the general case can be proved without Zorn's Lemma (or something equivalent). As a test case one could look for a Zornless proof for the Prüfer surface. Even simpler, one could look at a the disjoint union of an uncountable number of copies of Euclidean space.