Hi strasshgoa,
Good advice from Calin - my guess would be that you don't have a redirect in place for that, or that you may have some other canonical issue, perhaps caused by having written the same URL differently in a link. An example of this would be using both www.mysite.com and www.mysite.com/index.html in your code. While both call the same page, they are different URLs and therefore seen by the crawler as duplicate pages.
The easiest way to identify the problem is to click the blue links in the column to the right of the URL that has been identified as having a duplication issue in your Report. The number of URLs that have been identified as duplicates of the page will appear as a link and when you click the number you will see the list of URLs.
There is also a help page for each of the tools in the Pro App which you can access by clicking the tiny blue "? Help" link to the right of the page towards the top (directly opposite the summary link on the left of the page). The help page for Crawl Diagnostics is here.
Hope that helps,
Sha

