porlandi : I think I might have answered this on the ifinity forums, but in case you're a different person, my response is the same.  As already mentioned in this thread, it's not a really a big deal if you can create duplicate content by linking to someone else's page.  I can do it by linking to www.microsoft.com/en/us/default.aspx?bill-gates-is-rich (try it!)   Unless someone has a concerted campaign across many websites to do this, it's unlikely to cause any effect at all.  Search engines purposely do not allow others to cause damage to another site's rankings by nefarious purpose : otherwise there would be a massive industry in destructive linking going on.

If you do happen to get a non-existent Url linked by another page, then you can handle that one exception by either redirecting that url or issuing a 404.  For the rest of the time, it's not a big problem.  There are much bigger wins to be had in the SEO game than sending out 404's for pages which cause duplicate content by non-existent querystrings and extra path information.  All of the possible duplicate Urls for the home page (ie /, /default.aspx, /home/tabid/xx/default.aspx and /home.aspx) for a standard DNN install are far more of a problem than someone creating duplicate urls mischievously.

Having said that, it's not the first time I have heard this request, so I might build a module specifically to capture and stop everything but allowed querystrings/path extensions.