If you are concerned about duplicate content on your site, checkout both Yahoo! and Google. Yahoo! Site Explorer and Google Webmaster Tools have both got options where you can tell which parameters in URLS you wish to have ignored.
Google have quietly added the feature with little in the way of explanation. To access the feature, simply login to your account, go to Site Configuration then Settings – it is the last option on the page.
You can specify whether you want Google to ignore up to 15 specific parameters in your URL. This can result in more efficient crawling and fewer duplicate URLs, while helping to ensure that the information you need is preserved.
If your URLs include session IDs or source code then you may well have several URLs pointing to the same page. You can tell both Google and Yahoo! which of these codes to ignore.
Duplicate content may affect your search rankings, not through search engine penalties, but through diluted link juice. If your pages are only accessible through the one URL then, in theory, that URL will receive all the benefits of inbound links. If you have more than one URL, then each URL could be sharing those inbound links. If it’s an issue – fix it!