So what is duplicate content? To answer this question you need to know what unique content is and what it isn't and it's not what you think. Unique content is content that is that is different from what everyone else is saying. But the mistake people are making is they go find some site on the web and reword the information to make it "their own". It's unique but you are still saying the same thing everyone else is saying, aka, "me too marketing" or the "I want to say that too" theory.
Let's get something straight, the search engines want to see something of value that came strictly from you and no one else. This is what you need to focus on. You need to say something that someone else hasn't already said. If someone else already said it, than it's too late for you because in most cases the search engines already picked that up from other sites, especially the sites who are popular and those whose information is already being shared by social media outlets.
So the idea is to provide unique valuable content rather than just so called unique content. Also for example if someone is looking for an Italian restaurant, you can't just go and get information from an online directory and take the content about a popular restaurant and reword the stuff, you have to give the details about the restaurant and go above and beyond and talk about the stars that comes to the restaurant, talk about the area, and if the Italian restaurant has a movie theater underground, talk about that and get a bunch of unique pictures and put that on your site, (with permission of course). Most people aren't gonna do that. Give the details and make absolute sure you share information that other people aren't sharing.
Now that you understand what unique content is and that you should always be providing unique valuable content and something that no one else ever mentioned on any other website, you can fully understand what duplicate content is.
So what is duplicate content? This is the process where a blogger or site owner take full blown copies of someone else's work and put it on their blog or site without proper credit given back to that person's blog or website. Also when people take content that already exist in the internet world and put it on their blog. If that content was already out their first, that site will show up before anyone else who puts the same content up afterwards. Basically, duplicate content is another form of plagiarism. We can just call it plagiarized content or high-jacked content. That's all it is.
Now this is what duplicate content isn't. It isn't when a person takes a few sentences from some one else's blog post or article and post it on their blog post or article with some of their own unique content. Too many people get that wrong. It also is not when a person cites what some one else says while also providing unique value. Why, because you can share what someone else said on your blog as long as you give them proper credit. This method is known as content curation and this is actually better than saying "oh me too". You pretty much learnt that in school.
Bloggers and site owners get this stuff confused all the time. But that can stop today. As a matter of fact, below is a video from SEOmoz. Rand Fishkin is doing a segment called whiteboard Friday, which many of you may already know about. He is talking more on this subject and I believe it is very important for you to hear. Rand Fiskin clears up the confusion on duplicate and unique content. The video is titled: How Unique Content Needs to Be To Perform Well In the Search Engines? Take a look at it now.
Now doesn't that help you? It's really good info.