Duplicate content issues

Hey folks,

just joined here and know quite a bit about SEO already, implementing Drupal sine 4.6

One of my major issues & concerns still is, that altough we have path_alias and path_auto to create nice paths for us,
they are only aliases for the original nodes...

This means the aliases are STILL accessible, so we get multiple urls like these two in a simple node alias

site.com/my-nice-path
site.com/node/4711

effective today one could start to rank for both pages, as the "old" node path is not redirected properly (with a 301) to the alias. This could also mean that the "good" path (which does convert better, is better memorized ) could be burried in the SERPs
or the worst, both could be removed from the search results

Anyone out there to share my concerns?

best,christoph