>In my main page, I have a small JavaScript that opens another windows with web site's introduction page. In order to open this window when the main page opens, I put a call in the BODY's onload.
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>The problem is, I only want to open this page once per session so that when the same user revisits the main page in the same session it doesn't appear again. I'm using Session object to track every user so I can use Session('ViewIntroPage') or something to flag it but how/where can I call this so that it gets automatically run just the first time?

Check to see if the variable exists... if it does not, show the page - otherwise don't show it.