Microsoft, Yahoo and Google have all together launched a tag for solving problems with duplicate content. Instead of an ugly link something like this http://www.example.com/page.html?sid=asdf31415ihijkjuk454 you can use this in your code, in the HEAD part, to point out the canonical link:
<link rel=”canonical” value=”http://example.com/page.html”/>
Read about duplicate links at Google Webmaster Central blog, Yahoo Search blog, Live Search Webmaster Center blog. You also have a help instruction about canonical pages at Google help.