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Ed, there's no mistake in the article. {1,2,2} represents the same set as {1,2} because in sets repetitions are ignored. However, if these were multisets instead, then they would be different multisets because for multisets repetitions do matter. I'll use MathWorld as a source: "A set is a finite or infinite collection of objects in which order has no significance, and multiplicity is generally also ignored (unlike a list or multiset)". I think what might be causing confusion is the idea of enumerating a set, which is when you write out all the elements of a set with no repetitions. But generally you're free to write as many repetitions as you want, they don't change the actual set you're dealing with.

This edit should be reverted, but I agree it's useful to mention multisets at some point - preferably before the Russell's Paradox paragraph. -Foxtrot 14:07, 31 August 2008 (EDT)