Google Webmaster Tools - Duplicate content (slash and none slash version of a page)

Question

Google Webmaster Tools shows a lot duplicate content in the following pages: www.example.com/example www.example.com/example/

I do NOT offer two versions of any page on my website, when you type example.com/example you are REDIRECTED to example.com/example/. However, Webmaster Tools shows the both versions of all my pages, considering it to be a duplicate content.

How can I fix this?

Thanks!