If Duplicate content is SI

Selected answers from the Dumb SEO Questions Facebook & G+ community.

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  K Neeraj Kayastha: If Duplicate content is SIN, why Google lists these pages or site?
  
  I found many pages starting with There is a high possibility you found us through a search engine. And the fact you are on this page is good enough to assume that search engine optimization and found 60 unique pages of various websites.
  
  I am not talking about real SERP ranking, but why Google even consider to let these remain indexed!
  
  And what the content writers are doing? These results are of Internet Marketing Consulting Companies websites. ;So how these Consultants are doing SEO of clients' websites!
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  Deepak Gupta: Google is not even sure about its Duplicate content policy. Google bans only when someone SPAMS not when using duplicate content. You can use duplicate content within a limit - this is what my understanding is