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We have a client that has his products listed on 20+ different websites, including 4 of his own.  Also, he only has 1 of everything, so once he sells it then the product is gone. To battle this duplication issue, plus having a short internet lifespan of less than 4 weeks, I was wondering if it would be a good idea to canonical the products back to the category page.  Kind of like using canonical tags on your "used blue widget" and "used red widget" pages back to the "used widgets" page. Would this help with the duplicate content issues? Is this a proper use of a canonical?