If you have an URL like this:
If someone creates a link on his website, but changes the name like this
that would cause duplicate content. Is it nessecary to check if the name in the url is the same as the real name of the question and if not do a 301 redirect to the correct URL?
I see Stackoverflow.com doesn't do a 301 redirect, but just accepts any name in the URL. Can this cause duplicate content or your ranking to drop? Is SO not affected by this?
This won't cause duplicate content as long as you use a <link rel="canonical"> element. For example, this question's <link rel="canonical"> element is:
<link rel="canonical" href="http://stackoverflow.com/questions/3488655/seo-duplicate-content">
This element does not change even if you visit the page with a different slug at the end of the URL.
It tells Google that, even if it got to this page via a different URL, it's the same page as the canonical page (not a duplicate) and it should be counted as such.
More info: Official Google Webmaster Central Blog — Specify Your Canonical