[Zope] Zope Session

Hi all, i have a user with username say 'shamol'. I am loggin in with username 'shamol' in a browser in my site. Now i open another browser on the same machine or any other machine and try loggin in with same username 'shamol'. This second login with same username 'shamol' without logging off the previous login is successfull. I want to prevent this. My requirement is that any user should be able to login for first time and if user tries to login again without loggin off then user must be prevented by displaying necessary message. Any solution is most welcome, thanx, Ashish.