It's pretty self-explanatory. Duplicate content is duplicate content. For example; you copy and paste someone elses article into your blog. That is duplicate content. They'd probably have the upper-hand in the search engines because they had it first. I logically assume that it's a first-come-first-serve in search engines when it comes to duplicate content.
Well, I think even if it is your own and you place the same content on more than one of your own sites - it is still duplicate content, correct?
Correct. Duplicate content is duplicate content. Whether it be yours or someone elses, accidental or incidental.
Duplicate content has nothing to do with copy and paste activities, but can be still done. You fall in duplicate content whereas you: - Copy and paste content posted somewhere else. - Use articles already published elsewhere,( i.e. eZine Articles) regardless you are authorized to do so. - Write your own articles and post them repetitively in more than one website - Put exactly the same article in the main page or elsewhere within your main domain, but also in another area inside it.
In the field of search engine optimization (SEO), duplicate content refers to internet content that appears on multiple web pages and/or web sites. Search engines like Google and Yahoo! tend to frown upon it. If they come across two or more pages that have the same exact content, search engines will usually only display one of them in the search results list.