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