Why does it happen?
Content duplication is a very usual thing in SEO. Is one of the biggest, if not the biggest issue in SEO today.
It happens because either you include the same content in different pages or because your content management system duplicates the content automatically for you. This is specially true if your content management system uses clean URLs and dynamic URLs.
There are different kinds of duplication of content and different levels: The complete duplication (mirror pages or sites), the partial duplication (blocks of code), hidden duplication (changing a couple of words here and there).
How to prevent the content duplication?
There are several ways to prevent content duplication. Some involve configuration of your hosting environment, some others include just manual effort. Make sure you identify the original version of your content. Then block everything else with the same content.
How to check in Google for the duplication of content?
Do a search for some of your content in quotes that should appear only in one of your pages. Remove the ones that you don’t want indexed.