I observed that StackOverflow uses two types of links:

[Should I list PDFs in my sitemap file?](https://stackoverflow.com/questions/1072880/sitemap-xml)
and
[Should I list PDFs in my sitemap file?](https://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.