http://drupal.org/project/globalredirect

drush dl globalredirect

Global redirect makes sure that when people visit a page on your site, they see your latest beautiful alias, not the Drupalish internal path or your second-to-last alias.

For instance, even if someone links to node/23 directly, they will be redirected to the aliased version, point/official-release-drupal-2-0

(Go to the module's project page for a 7 step description.)