Google's head of search spam, Matt Cutts said that 25% to 30% of the web content is duplicative. In the recent webmaster video, Cutts has talked about duplicate content and its impact in Google's search results.
More than one-quarter of the web pages and content is duplicative or repetitive. But, Cutts said there's nothing to worry about as Google doesn't consider duplicate content as spam. However, it is true that Google will show only one of those pages in search results, which may look like a penalty to you, in case your page is not chosen.
Cutts revealed that Google takes all the duplicate pages/content and groups it into a cluster & will show only the best results from the cluster. However, Cutts said that Google has the right to penalize a website, which has excessively duplicate content present in a manipulative manner.
Take a look at the video: http://www.youtube.com/watch?v=mQZY7EmjbMA25-30% of the Content on Web is Duplicate but That's Okay: Matt Cutts!,