There is no better way to create an infinite amount of duplicate content on your site than to force session IDs onto each visitor (and search engine). Typically, session IDs are used for tracking a single visitor's navigation path through the site, including the adding or removing products from the shopping cart. They are great for tracking purposes, but really, really bad for search engines and inbound linking.

Ok, first of all, that's a totally crappy URL shown above, but aside from that, tacked on at the end there is the session ID. Both URLs are the same, all except the session ID. I was able to open the exact same page, with the unique ID simply by starting a new browsers session. The problem is that the session ID constitutes a completely different URL. It's not an issue for the visitor, but it is for the search engines.

Since a new session ID is attached with each new visit, each time the search engine comes around they are essentially fed all new URLs. If you have only a ten page site, the second time the search engines visit they add the "new" 10 pages to the index, for a total of 20 pages. When they come around a third time they now have 30 pages in their index. Once they start analyzing these pages they find page after page after page of duplication.

An additional problem arises as site visitors start bookmarking and linking to your site. Every link they add contains their very own session ID. The search engines follow that link to your site and now you've got another 10 pages of duplication. If they follow another link to your site, that's 10 more. You starting to see where this is going? Essentially you can turn a 10 page site into endless duplications.

Even with a small site you can see why the search engines would stop coming around. But if you have a site with hundreds, or even thousands of products, you find two things happen. 1) The search engines will stop spidering new pages because there is just too much duplication. 2) The engines will start dropping pages out of the index altogether.

There are content management systems that will allow you to withhold the session IDs from search engines. While this is a good option it still has the potential of creating problems with inbound links. Each link will still pass value to the URL with the session. It'll be up to the search engine to make a determination if the URL with the session and the URL without are the same.

The only guaranteed protection is not to do it at all. There are alternate means of tracking users for whatever reason. Avoiding session IDs completely ensures that you don't open yourself up to inadvertent site duplication.

This article is part of a series on duplicate content. Follow the links below to read more:

1. Theories in Duplicate Content Penalties
2. How Poor Product Categorization Creates Duplicate Content and Frustrates Your Shoppers
3. Redirecting Alternate Domains to Prevent Duplicate Content
4. Preventing Secure & Non-Secure Site Duplication
5. Why Session ID's And Search Engines Don't Get Along (Hint: It's a Duplicate Content Thing)
6. What Does a Title Tag, Title Tag and Title Tag Have In Common?
7. How to Create Printer Friendly Pages Without Creating Duplicate Content
8. How to Use Your WWW. to Prevent Duplicate Content