Duplicate content

Duplicate content generally refers to substantive blocks of content within or across domains that either completely match other content or are appreciably similar. Mostly, this is not deceptive in origin. Examples of non-malicious duplicate content could include:
Discussion forums that can generate both regular and stripped-down pages targeted at mobile devices
Store items shown or linked via multiple distinct URLs
Printer-only versions of web pages
However, in some cases, content is deliberately duplicated across domains in an attempt to manipulate search engine rankings or win more traffic. Deceptive practices like this can result in a poor user experience, when a visitor sees substantially the same content repeated within a set of search results.
Google tries hard to index and show pages with distinct information. This filtering means, for instance, that if your site has a "regular" and "printer" version of each article, and neither of these is blocked in robots.txt or with a noindex meta tag, we'll choose one of them to list. In the rare cases in which Google perceives that duplicate content may be shown with intent to manipulate our rankings and deceive our users, we'll also make appropriate adjustments in the indexing and ranking of the sites involved. As a result, the ranking of the site may suffer, or the site might be removed entirely from the Google index, in which case it will no longer appear in search results.

There are some steps you can take to proactively address duplicate content issues, and ensure that visitors see the content you want them to.
Consider blocking pages from indexing: Rather than letting Google's algorithms determine the "best" version of a document, you may wish to help guide us to your preferred version. For instance, if you don't want us to index the printer versions of your site's articles, disallow those directories or make use of regular expressions in your robots.txt file.
Use 301s: If you've restructured your site, use 301 redirects ("RedirectPermanent") in your .htaccess file to smartly redirect users, Googlebot, and other spiders. (In Apache, you can do this with an .htaccess file; in IIS, you can do this through the administrative console.)
Be consistent: Try to keep your internal linking consistent. For example, don't link to http://www.example.com/page/ and http://www.example.com/page and http://www.example.com/page/index.htm.

Use top-level domains: To help us serve the most appropriate version of a document, use top-level domains whenever possible to handle country-specific content. We're more likely to know that www.example.de contains Germany-focused content, for instance, than www.example.com/de or de.example.com.

Syndicate carefully: If you syndicate your content on other sites, Google will always show the version we think is most appropriate for users in each given search, which may or may not be the version you'd prefer. However, it is helpful to ensure that each site on which your content is syndicated includes a link back to your original article. You can also ask those who use your syndicated material to block the version on their sites with robots.txt.

Use webmaster tools to tell us how you prefer your site to be indexed: You can tell Google your preferred domain (for example, www.example.com or http://example.com).
Minimize boilerplate repetition: For instance, instead of including lengthy copyright text on the bottom of every page, include a very brief summary and then link to a page with more details.
Avoid publishing stubs: Users don't like seeing "empty" pages, so avoid placeholders where possible. For example, don't publish pages for which you don't yet have real content. If you do create placeholder pages, use robots.txt to block these from being crawled.

Understand your content management system: Make sure you're familiar with how content is displayed on your web site. Blogs, forums, and related systems often show the same content in multiple formats. For example, a blog entry may appear on the home page of a blog, in an archive page, and in a page of other entries with the same label.

Minimize similar content: If you have many pages that are similar, consider expanding each page or consolidating the pages into one. For instance, if you have a travel site with separate pages for two cities, but the same information on both pages, you could either merge the pages into one page about both cities or you could expand each page to contain unique content about each city.

Duplicate content on a site is not grounds for action on that site unless it appears that the intent of the duplicate content is to be deceptive and manipulate search engine results. If your site suffers from duplicate content issues, and you don't follow the advice listed above, we do a good job of choosing a version of the content to show in our search results.