Site owners will now receive alerts when Google detects duplicate content on another domain through their Webmaster Tools platform. The new alerts will help webmasters be proactive about sites that copy or republish their work and in some cases discover issues within their own sites.
It's no secret that copied content is a huge problem online. We normally associate the problem with sites made for Adsense that scrape content to build pages but there have been cases of established sites finding and re-posting content without crediting the original source and in some cases ranking higher than the original material. The Panda update was supposed to fix some of the duplicate content issues but some webmasters are still complaining that copied content outranks them with malicious site owners changing publishing dates and timestamps to try and confuse crawlers.
Not all duplicate content occurs as a result of malicious activity. Sites with poor canonicalization could suffer from different URLs resolving to the same content and companies with multiple websites often suffer duplicate content issues on about, services and contact us pages.
Many sites use services like Copyscape to monitor their content and Google's new alerts should provide a complementary service to existing monitoring solutions. If someone is copying your content, filing a DMCA is the most effective method of making sure you are properly credited and the offending material is removed. We also recommend immediately promoting any new content that's published in order to give search engines a better opportunity of finding your work before it ends up on another site.