what is duplicate content?

                    Duplicate Content

In SEO (search engine optimisation) terminology, it is content (or text) that has been copied or reused from other Web pages. Duplicate content is often used to help boost keyword density, however, some search engines, including Google, filters duplicate text and may penalise your site, resulting in a lower keyphrase position, when you use duplicate content.

The Three Biggest Issues with Duplicate Content
1.Search engines don't know which version(s) to include/exclude from their indices
2.Search engines don't know whether to direct the link metrics (trust, authority, anchor text, link juice, etc.) to one page, or keep it separated between multiple versions.
3.Search engines don't know which version(s) to rank for query 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.

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 a .htaccess file; in IIS, you can do this through the administrative console.)

Be consistent: 

Try to keep you 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 index meta tag to prevent search engines from indexing their version of the content.

Use Search Console to tellus 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).

Minimise 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 no index 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.

Minimise 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.