We are experiencing the same issue -- the system allows different documents in the same library to receive the same URL. It then seems to be somewhat random which version of the document is displayed when someone visits that URL.
Because we have a large number of users who upload to our site, we're also looking for the solution that prevents this from happening in the first place. At the moment, the workaround is not intuitive for a very basic user and can create a file management nightmare in the back end.
We're running 5.1