Matt Cutts, today in his recent video explained how Google handles duplicate content issues for internal and external duplication as well as its effect on search engine ranking results.
Matt Said that it’s common and also Google realizes that normally on the web 25% to 30% of content are duplicated. Google does not consider those contents as duplicate which links to another post or resource with some introduction or piece of content because such content happens automatically on the web.
Matt further explained that Google does not take all duplicate content as spam content, and Google will consider all these as spam and penalize them, he will be hurting his own search results quality. In the actual about detection process of duplicate contents, Google checks each piece of the same content and groups theme to determine spammy and duplication nature of contents.
Matt also warned the webmaster by saying that Google reserve the right for deciding what is duplication or not. Matt says that genuine contents are of manipulative nature whose duplication happens automatically and can be seen as many pieces of the same content on the web.
In brief, Matt wants to explain that Google has got technology for detecting what is actual spam and duplicate content published intently and what is genuine content. Google technology is too advanced to find such duplicate content and penalize it.