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I would slightly disagree with you on one point - the need to check every line. There are high probabilities, given the vastness of the web, of individual lines being found somewhere else. Where duplicate content becomes an issue is if a decent-sized section is found elsewhere or a large proportion of your page is not unique. So for an individual page, maybe checking half paragraphs wouldn't take too long, unless you have to do it site-wide. It hadn't come across Copyscape, so that's helpful for me going forward. Thanks!