Session IDs are very common with Magento and they can cause severe duplicate content issues, as there’s no limit to the number of duplicated URLs that can be generated. Session IDs are used to track a user’s session and they’re usually generated via the checkout, to track what items have been added to cart. A session ID will be appended to the end of URLs, here’s an example: domain.com/page?__SID=df23n54jtklg.

The best way to deal with this is to properly fix the issue, which will require development resource. If you’re not in a position to fix this, I would recommend blocking the URLs via the robots.txt or assigning meta robots rules (noindex, follow) – I would also recommend providing instructions to Google via the parameter handling options in Google Webmaster Tools.

You should also block you checkout pages so that crawlers won’t access pages with the session ID appended to the end of the URL.