Duplicate content may be intentional or not. Sometimes the search engines are smart enough to know which ones are intended for usability purposes and which ones are a ploy to manipulate search engines.
To get the full details on duplicate content,there is nothing much I would write that has not been discussed in this Google Support article on Duplicate content.
Make sure you click the link and read the entire article. This is a good resource that I believe I should share with you guys. So you do not create two blogs and run similar content hoping that you will manipulate search engines . (Sounds out of the blues but there are people who actually do this!)
The Link again: https://support.google.com/webmasters/answer/66359?hl=en