The big 3 announced support for internal duplicate content. If you have a site with more than one way to get to a page, there is a new method for declaring the correct page. Simple to do in Miva Merchant, and probably other programs, the tag doesn’t have to be followed of course. Perhaps in the future the search spiders will restrict themselves. Think of it as a 301 redirect.
There are already WordPress and other plugins to automate this process (I told you it was simple) and there are bound to be others jumping on board. Not earth shattering and some are skeptical, but well worth a read.