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Ben, it sounds like it should work--but there are preferred methods that I would try before you use nofollow. Ideally, you should use the canonical tag on those pages, and not the nofollow tag. If someone were to link to one of those pages (sorting options), then you wouldn't get any credit passed onto the main page of the category. Using a canonical tag accomplishes what you're trying to do, and has greater benefits in the long run. Another option would be to get rid of the sorting options entirely, and not allow anyone to sort them (and remove the links to those sorting options, as well. If the sorting options don't exist, then there won't be any duplicate content generated.