Status_203 - 8:45 am on Apr 22, 2010 (gmt 0)
A url needs a protocol (a web browser will assume http if not supplied), a domain and a path. www.example.com has no path. www.example.com/ is a request for the root "/" at www.example.com .
If I attempt to go to www.google.com in my browser then I end up at www.google.com/ . So where is this happening. Using Firefox with the Live HTTP Headers add on shows me making no request for www.google.com but immediately requesting www.google.com/ . So it looks like the browser recognises that www.google.com is not a valid url but realises that in this case what is actually wanted most often is the root path of the domain "/" and so defaults this part of the path just as it defaults the "http://" part.
It would beggar belief if the search engines did not realise that http:/www.example.com is not a valid url, so I wouldn't worry about duplicate content issues in this case.