One of the questions I get most often regarding SEO is “Is duplicate content bad for my search engine rankings?” The answer to this question is…it depends.
Are you only providing duplicate content on your website? Or, is the majority of your content duplicate content? If this is the case, then yes, you are likely to get penalized for it in the sense that Google will choose not to show your site in the search engine results.
If you are adding value to the duplicate content then you will be fine. For example, may you have taken a quote from another blog, or decided to cite content from another website, then the answer is no, the duplicate content will not hurt your search engine rankings.
In the video below, Matt Cutts specifically talks about how Google treats duplicate content. Watch the video, but basically Google will group all the pages with duplicate content and then decide which page it should include in the search results. You can see examples of this when Google will give you a message that it omitted similar content and gives you an option to click a link to see that content.
Check out this video of Matt Cutts speaking about how Google handles duplicate content: