Am I Famous Now…

In my recent 5 SEO Quickies for your Blog, Joefish touched the topic of duplicate content in the comments.

Changing the slug on posts that have already been crawled could lead to searchers getting 404s, plus there’s the possibility your ranking might actually be lowered if the search engine thinks you’re cheating by having two posts with identical content

I agree that one should be careful when changing the url of an old post, but not for mentioned reason. The danger when changing the url of an older entry isn’t a matter of duplicate content, but your entry might lose its ranking in the Index of Google (and other SEs).
Many times when you think you can actually improve the ranking of your site/url/entry, you will enter a larger market and face more competition. Sometimes it can bring you more visitors to chose for the smaller market, or even the odd typo can bring you many visitors.

Never change a winning team

If the url you have chosen does bring you visitors, there is no need to change it. If not you might give it a try. Before doing this you should use the Google Webmaster Console.
Under webmaster tools you will find the option statistics after having clicked the domain you want to manage. In the statistics you will find under query stats which are your the most popular queries and where you rank in Google for those search terms.
Then you can use a tool such as Word Tracker or have a look at this entry.

To resume : Think before you change the url of an entry. You might get less visitors afterwards.

If you want to know what duplicate content from the view of Google is, visit the Official Google Webmaster Blog.
More on duplicate content, 404 and using redirect next time. ;-)