We have dealt with duplicate content with Joomla 2.5 on our video How to Solve Duplicate URL's
With Joomla 3 these measures no longer work, leaving rogue trail slashes URLs.
We have dealt with it by using 301 redirects using a component called RSseo, but it can also be acomplished using Joomla's own Redirect Component, just make sure to activate the plugin.
We did a complete test today starting from the scratch, we've installed a 3.3 template, then upgraded the version of joomla to the lastest version Joomla 3.4! and the results were encouraging, in this version of Joomla 3.4 those trail slashes didn't show up..