I think of my own experiences of Duplicate Content issues and normally would think of ripped off content from a client's site that appears on another site.  Or maybe, a client, just not knowing, makes copies of many of their pages and puts them up on the web not realizing there might be a problem.

However, I've never seen duplicate content  explained as simply as Duplicate Content Issues Caused By URL Parameters from Just Like Mike blog.

"..If there is more than one URL that you can use to display the EXACT same page on your website, search engines will consider that you have duplicate content.  This can negatively impact you."

Once, a client had several sub account pages that copied the contents of their main pages, and they did not realize anything was wrong with that - until someone told them.  Once we made sure the duplicate pages were no longer being indexed, the site's rankings began to steadily improve.

In another case, a client made 12 identical copies of their site, using different domains which created URLs that were different but accessed almost identical content.  I told the client this was a bad idea and they put that project on mothballs.

The post goes on to say:

"..So, if you have dynamically created pages, test them out to see if there is more than 1 URL that you can use to display the exact same page.  If there is, work out how to consolidate it down to one page.

ALSO, in the process of merging your URLs, don’t loose your existing search engine index link love - no matter how poor it is.  Use 301 redirects to forward the dupe URL to the correct URL.  Search engines will see this, and adjust themselves accordingly.
This tip doesn’t help you get a better search engine position, it just helps prevent you getting a worse position than you would."