I just found that use of session ID in ecommerce sites can cause a sitewide duplicate content issue. For example, you may have noticed there are different URls in Google index for your the same product URL:

http://www.yourwebsite.com/file1.html?osCsid=2e1349d6866367e48ec9b6f4a7c665f8

http://www.yourwebsite.com/file1.html?osCsid=2dsdfdsd6866367e48ec9b6f578dsdcxs

http://www.yourwebsite.com/file1.html?osCsid=3dsdsf3dsdfds6367e48ec9b6f4a7csdsf

They all belong to the same file , but containing session ID. When Googlebot visits your site, and your website gives Googlebot session ID. What will happen???

Google sees those new URLs, crawls and indexed it , yet it is already indexed before but using the old session ID Googlebot has.

How to correct this issue:

1. Block session ID using robots.txt.
2. Change all navigation links to permanent and static URLS, especially links to your sitemap.html should be using static ones (no session ID).
3. The XML sitemap you have submitted to Google webmaster tools should not be using session IDs.
4. Once the session ID free is being indexed by GOogle, remove all your indexed session ID URLs using Google webmaster tools URL removal tool.

That’s all folks. :)