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Thanks Paul

Like Deacyde I use the content in quotes in Google itself "to test if this is duplicate." But I also use a couple tools: http://www.copyscape.com/ http://www.siteliner.com/ http://www.webconfs.com/similar-page-checker.php http://www.plagspotter.com/ (undergoing a bit of renovation currently) If you wanted to build this into your own tool, PlagTracker has an API http://www.plagtracker.com/api.html (As do a few of the ones above.)