Duplicate content is always a subject that interests us, because we (webmasters) are afraid of it. We are afraid to get our content duplicated, we are afraid to get caught by google for duplicate content either by articles that we get from an article site of for having duplicate content on our own websites.
Copyscape
I have mentioned before an excellent tool to seek for duplicate content that is called Copyscape. I used it every time i write an article, or every time a change the article structure. It only gives me 10 results but that is more than enough.