Google provides tips on duplicate content

Duplicate content is an important issue and something that can have an adverse effect on a website’s search engine rankings.
So lots of site owners will be pleased to hear that Google has provided some tips on how to address it.
According to Sven Naumann on Google's Webmaster Central Blog, there are two main types of duplicate content:
Duplicate content within one website
This is often unintentional and can be the result of sites having pages for similar products where the content has been only slightly changed, or because landing pages have been created for PPC campaigns.
In this case, Google recommends that webmasters include the preferred version of the URL on their sitemap file, which will help the search engine's crawlers find the best version.
Duplicate content across domains
This refers to content identical to that on your website appearing on third party domains, often when sites use scrapers to copy your text and use it to push themselves up the rankings.
Naumann claims that Google manages to determine the original source of the content "in most cases", and that having your content copied shouldn’t impact on your search rankings.
He offers the following tips if sites with scraped content are ranking higher than the original website:

• Make sure your site’s content is being crawled by Google.
  
  
• Check the Sitemap file to see if you made changes for the particular content which has been scraped.
  
  
• Make sure your site is in line with Google’s webmaster guidelines.