Looking at your site I see that your Email a friend script is opening a brand new page and passing through the product ID. To stop the page being indexed you can add the following line to the robots.txt
Disallow: /EmailaFriend.asp
afte the line that looks something like
User-Agent: *
Heres some light reading on the robots.txt file
http://www.seomoz.org/learn-seo/robotstxt
BE CAUTIOUS! if you dont know what your doing seek further assistance as you could do some bad things in the robots.txt.
Whilst the solution above will probably solve your duplicate page issue. I think there is room for improvement here.
Email a friend scripts needn't take the user away from the product they are viewing. You could open the email a friend script in a new window, or even better get rid of the Email a friend button altogether and make your social sharing buttons bigger and more prominent. Also, consider giving the users a Call to Action to sharing the product via the social buttons.
Hope this helps.
Aran