If you website has duplicate content, here is a good article on how to Getting Rid of Duplicate Content Issues Once and For All.
Particularly, if you have exact duplicate content, and you just don't want to change your website, you can:
- For Yahoo!, you can use the Yahoo Site Explorer to do dynamic URL rewriting. Login to your Yahoo Site Explorer, click the link about your site, and then find "Actions" -> "Dynamic URLs" in the left sidebar. Then enter the URL parameters one by one to instruct the slurp to remove the parameter in the URL.
- For Google, you can use the Google Webmaster Tools to specify your canonical. That is, to add in your page's <head> part something like
<link rel="canonical" href="http://www.example.com/product.php?item=the-final-page" />
in every page that is a duplicate to the URL as specified in the above <link> tag.