title: How does this site know that I am opening another tab?
body: <p>When I login to a site, which is a "learning system" at my university, I have found that I cannot open a new tab with the same site open. When I do so, somehow the site is aware and displays the following message?</p>
  
  <p><img src="https://i.stack.imgur.com/QpxK8.png" alt="Session already running?"></p>
  
  <p>How on earth does the site know what tabs I have open on my computer? As far as I know, the front-end code shouldn't have any access to my private browser information. What accessible information could this site be using to determine that I have another tab open already?</p>
  
  <p>I am accessing the site using a private computer, and the site being accessed isn't on a local network, it is being accessed through the internet. Therefore there is no internal monitoring software that could be causing this. I am using Google Chrome 24 Beta for Mac.</p>