Matt said that somewhere between 25% to 30% of the content on the web is duplicative. Of all the web pages and content across the internet, over one-quarter of it is repetitive or duplicative.
Matt Cutts says you don’t have to worry about it. Google doesn't treat duplicate content as spam. It is true that Google only wants to show one of those pages in their search results, which may feel like a penalty if your content is not chosen — but it is not.
Google takes all the duplicates and groups them into a cluster. Then Google will show the best of the results in that cluster.
Matt Cutts did say Google does reserve the right to penalize a site that is excessively duplicating content, in a manipulative manner. But overall, duplicate content is normal and not spam.