John Mueller from Google has a wonderful response to a question about duplicate content and how to measure how big of an issue it is on your web site. He posted it in the Google Webmaster Help forums.

In short, he called duplicate content issues an issue SEOs can work on, on one of those rainy days. He said Google does a good job dealing with it, and with all the issues you probably have on your site, duplicate content is probably not a primary issue relative to what else you have on your to do list.

Here is what John wrote:

So much here, I enjoyed reading it and wanted to share it with you all.

Forum discussion at Google Webmaster Help.