Duplicate content issues are one of those plaguing issues that SEOs have to deal with often. Often, Webmasters are unaware they have a duplicate content issue until it is too late. Yes, by too late I mean, your pages are dwindling down in the index and your rankings suffer. So what can you do?
Well, there is a new tool out named Site Wide Duplicate Content Analyzer;
This tool crawls your entire site and then analyzes all your pages for duplicate content. It shows similarity percentage among all pages on your site, so you can see what pages are similar enough to trigger a flag in major search engines and consequently they can penalize your site for duplicate content.
I didn't try it out myself, but give it a whirl. Any comments or feedback, discuss at Search Engine Roundtable Forums.