Duplicate content and keyword in URL

Question

I observed that StackOverflow uses two types of links:

http://stackoverflow.com/questions/1072880/sitemap-xml and http://stackoverflow.com/questions/1072880

for the same question. The idea is clear: add keywords into URL and have SE pick up the page faster.

But shouldn't Google punish for the duplicate content in this case? I'm trying to understand what is more helpful since we have a similar situation on our site.

Answer 1

Source code has the answer.

<link rel="canonical" href="http://stackoverflow.com/questions/1072880/sitemap-xml"&gt;
    <link rel="alternate" type="application/atom+xml" title="Feed for question 'Sitemap xml'" href="/feeds/question/1072880">

rel = alternate/canonical

Answer 2

The idea is clear: add keywords into URL and have SE pick up the page faster.

It actually has nothing to do with the speed of indexing, however, it does with ranking.

As pointed above, best approach when dealing with duplicated content inside the same domain is putting a canonical element pointing to the preferred URL.