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Having a lot of duplicate content will make you vulnerable to Google lowering your rankings sitewide as they don't want sites with thousands of "thin" pages to rank well. You can focus on improving the content on your most important or popular pages and expand from there. Also, if you can encourage customers to review products that can be a great way to generate a lot of unique content in an inexpensive and scalable way (for example SEOMoz encourages UGC (user-generated-content) by awarding points and maintaining a leaderboard of top contributors).