A dupe made it through today with identical URLs; but a special character was URL encoded using 2 different capitalizations:
http://steveblank.com/2010/10/13/too-young-to-know-it-can%E2%80%99t-be-done/
http://steveblank.com/2010/10/13/too-young-to-know-it-can%e2%80%99t-be-done/
(There is an "E" that was capitalized in the first submission.)
I tested this by submitting the story a third time, and capitalizing a "T" that wasn't capitalized in either submission and it made it through the dupe check.