Duplicated content is actually not a new issue and every SEO should know about this problem. The best example of how content may be duplicated, is www-problem. If you don’t have a 301-Reditect of domain.de to www.domain.de, then the your website is reachable on both addresses. A result of this problem is a great quantity of same pages with the same content. Although we think that it is unimportant, Google, MSN and Yahoo have a different opinion. Therefore it is essential to solve this problem and not to ignore it.
But there are cases where it is inevitable to solve this problem on the way that I described. For example, if a web site offers a “Print Preview” or “view for mobile devices”. Previously, it was so that Google could decide which of the pages with the same content it takes into the index.
This is now different. The largest search engine operators have understood this problem and offer a remedy for webmasters who can’t avoid to use duplicated content. Now you can use the link with the attribute rel = “cannonical”. That means that you tell to a search engine that this link has more priority than other with the same content.
Source: http://www.mattcutts.com/blog/canonical-link-tag/
