Session IDs enable you to track user movements on your website. Session information is usually stored in cookies.
What are Session IDs?
A session ID is a unique identification number that is generated by a website’s server. Session IDs are assigned to site users when they visit a site for the first time.
Website operators can also use these identification numbers to track how site visitors use their websites within a defined period of time (session).
How are Session IDs stored?
The visitor’s click path can thereby be traced later on by website operators when they analyse the server logs.
Parameterised Session IDs and Duplicate Content
If session IDs are added as parameters to page URLs, Googlebot will receive a new ID in the URL whenever a page receives a visit from a user.
If the URLs with these added parameters then also respond with a status code 200, this signals to Google that the page under that specific URL exists and can be accessed by web users.
This results in large numbers of pages being added to the Google index that all have the same content – a typical case of duplicate content that is best avoided.
If your system relies on writing session IDs in the URL, it is advised that you use a canonical tag. This should be installed on every page that uses a parameter, and it should point to the original version of the page.