Google generally defines Duplicate Content as “substantive blocks of content within or across domains that either completely matches other content or are appreciably similar”.
There are many tips suggested by Google itself and other search engine optimization companies in order to avoid Duplicate Content in SEO such as:
- Use appropriate methods to block duplicate content
Instead of depending on Google’s algorithms to determine the "best" version of a document, you may guide Google to your preferred version.
- Use Meta Robots: NoIndex/Follow tag to eliminate Duplicate Content
Using the noindex value will tell the search engines not to index the page; thus, eliminating duplicate content while the follow value will tell the search engines to still follow the links found on this page; thus, still passing around link juice.
- Use 301 redirects ("RedirectPermanent") in your .htaccess File
It's recommended to use 310 Redirect if you have restructured your site in order to redirect users, the Googlebot, and other spiders alike from your old web page to a new web page. A link going to a page that has 301 redirects will be regarded as a link of the destination page of the redirect.
- Use Canonical Link Tag
If your site contains multiple pages with largely identical content, use the canonical link tag in order to indicate your preferred URL to Google and not index the duplicate content page. The only thing that will appear in the search engine index is the destination page. All the links that are going to the duplicate content pages will be considered as links of the main content page.
- Be Consistent When Choosing your Landing Page for your Link Building
Make an effort to be consistent in your internal linking. Avoid linking to /page/ and /page and /page/index.htm.
- Use Top Level Domains
Use top level domains or TLDs whenever possible in order to handle country-specific content to assist Google to serve the most appropriate version of a document.
- Make Sure to Syndicate Carefully
Remember to include a link back to the original article on each syndicated article that you had used if you syndicate your content on other sites.
- Remember to Use the Preferred Domain Feature of Webmaster Tools
Make sure to let Google know which way you prefer your site to be indexed if other sites that have link to yours are using both the www and non-www version of your URLs.
- Reduce the Use of Boilerplate Repetition
For example, use a very brief summary and then link to a page with more details instead of including lengthy copyright text on the bottom of every page. To specify how you would like Google to treat URL parameters, utilize Google’s Parameter Handling tool.
Don’t worry too much about other sites duplicating your content. Though it is infuriating, it is highly unlikely that such sites can negatively impact your site's page ranking in Google.