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One static URL that can be found by your users from within any of the categories that the product is in would probably be best. That way there is no chance of duplicate content issues if the search engines were to find both and not resolve the canonical tag. Really, I think it could go either way. Whichever one is easiest to implement in your particular situation, unless there are a lot of inbound links to your products. If that is the case, changing the URLs would require 301 redirects from the old URLs.