Avoid Duplicate Content and Recover from Google Penguin Update

Duplicate content generally refers to substantive blocks of content within or across domains that either completely match other content or are appreciably similar. After Google penguin update you should take care of duplicate content or else the site ranking will go down.

How to Duplicate Content

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 http://www.example.de contains Germany-focused content, for instance, than http://www.example.com/de or http://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 use the noindex meta tag to prevent search engines from indexing their version of the content.

Use Webmaster Tools to tell us how you prefer your site to be indexed: You can tell Google your preferred domain (for example, http://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. In addition, you can use the Parameter Handling tool to specify how you would like Google to treat URL parameters.

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 the noindex meta tag to block these pages from being indexed.

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

[source : Google Support]