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Using site:yourdomain.com in Google, and then going to the end of the results and telling it to show you all of the results, is a good first start. It should get you enough to get an idea of why there are duplicated pages. The Moz crawl can also help you figure it out, as often with ecommerce you'll have URLs for sorting products by price, name, pagination parameters, etc. We'll throw up a flag when we see a bunch of duplicate content or duplicate titles. Also look for the easy stuff, such as non-www doesn't direct to www. Fix that, and you've cut your pages in half.