I know this question has been asked before but I have not found a complete answer. When the user selects duplicate tab in IE it appears the current tab gets its url(with cookless session id) from the current window. Then the two tabs are sharing session values.
I have tried checking the referrer for null, but on the Duplicate Tab command in IE that value is set to the current tab.
The only workaround I see is to get rid of session.