I was under the impression that google/yahoo/msn would realize the two are the same page but have seen on a few sites lately both examples being indexed and ranking. Should I be concerned? An example: http://www.mattcutts.com/blog/asp-net-2-and-url-rewriting-sometimes-harmful is the same as http://www.mattcutts.com/blog/asp-net-2-and-url-rewriting-sometimes-harmful/ and if the non "/" wasn't indexed before it will be now for sure. Is this something we should be rewriting to avoid duplicate content?
They aren't the same thing though. Technically you could put different content at each URL. If you want Google to know it's the same, one should redirect to the other.