Duplicate content refers to that content which is copied from some other site and that content is posted by some other person on his site. Duplication is a spam act. As you can understand from Google definition, Google identifies two types of instances of duplicate content. The issue for duplicate content is real one of the webmasters. Having a duplicate content will not penalize a site in any way. Google does not spam your site if you are using duplicate content but your site cannot see the light of day in Google. Here the question arise is How To Use Google For Duplicate Content Detection?
If you look on some standard sites, it has usually a header, a footer and a side bar. CMS allow you to show your most recent posts or your most popular posts on your homepage. When the bots crawl your site then this content is present several times on your site. It is not included in duplicate content. This type of duplicate content cannot hurt your SEO. Here you can find information about How To Use Google For Duplicate Content Detection.
In some of the cases, the content is deliberately duplicated across domains in an attempt to manipulate search engine rankings. Here you can find some step that you can take to address duplicate content issues.
Use 301’s
You can use 301’s redirects in your .htaccess file to redirect users and Googlebot. You can find informative data about How To Use Google For Duplicate Content Detection.
Use Top Level Domains
You should use top level domains in your content. Use top level domains whenever possible to handle country specific content.
Understand Your Content Management system
You should make sure that you are familiar with how the content is displayed on your website. Many blogs, forums and such related systems often show the same content in multiple formats. You can take the example of a blog entry appeared on the homepage of a blog. If you want to know about duplicate content detection then you can easily find data from this topic How To Use Google For Duplicate Content Detection.
Minimize Similar Content
If you have many pages that have similar data then you should display these pages into one. Google does not recommend blocking crawler access to duplicate content on your website.