There are some steps you can take to proactively address duplicate content issues, and ensure that visitors see the content you want them to. (Remember that again Google is thinking MORE in terms of duplicate content issue on YOUR OWN site rather than other ones, since those others Google can handle quite easily by itself).

• 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.However, if our review indicated that you engaged in deceptive practices and your site has been removed from our search results, review your site carefully. If your site has been removed from our search results, review our webmaster guidelines for more information. Once you’ve made your changes and are confident that your site no longer violates our guidelines, submit your site for reconsideration.
• Don’t fret too much about sites that scrape (misappropriate and republish) your content. Though annoying, it’s highly unlikely that such sites can negatively impact your site’s presence in Google. If you do spot a case that’s particularly frustrating, you are welcome to file a DMCA request to claim ownership of the content and have us deal with the rogue site.
• If you find that another site is duplicating your content by scraping (misappropriating and republishing) it, it’s unlikely that this will negatively impact your site’s ranking in Google search results pages. If you do spot a case that’s particularly frustrating, you are welcome to file a DMCA request to claim ownership of the content and request removal of the other site from Google’s index.
• Even with that, note that we’ll always show the (unblocked) version we think is most appropriate for users in each given search, which may or may not be the version you’d prefer.
• 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.and then look well inside your own site:
• 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.
• 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.
• 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.
• 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).
• 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.

(Sources: Google Advice on Duplicate Content

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