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It's SEO best practice; doesn't mean that it won't effect something, but it's in your best interest. I think this Moz blog can say it waaaay better than I: http://www.seomoz.org/learn-seo/duplicate-content

You can use crawlers like xenu or screaming frog (http://www.screamingfrog.co.uk/seo-spider/)

Thank you sooo much! You rock!!