According to Google’s Webmaster Guidelines:
“Allow search bots to crawl your sites without session IDs or arguments that track their path through the site. These techniques are useful for tracking individual user behavior, but the access pattern of bots is entirely different. Using these techniques may result in incomplete indexing of your site, as bots may not be able to eliminate URLs that look different but actually point to the same page.”

This basically implies that duplicate copies of a web page may get indexed when using sessions IDs.