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How long does it take before it removes most of the duplicated search result pages from the index? Every site is different but I have seen it take 6 - 9 months for pages to drop out. Is it still crawling those pages but has not fully decided to remove most of them? It's possible. As Gaston has already pointed out, search engines will need to access those files again to see you want them noindexed. How bad is this for SEO? It temporarily dilutes the amount of SEO equity available to flow to pages you DO want indexed.