Tidy website links = duplicate content?

Up to now I’ve always preferred to check “tidy website links” in order to make urls shorter and more understandable (e.g. www.frieauff.com/about seems easier than www.frieauff.com/about/index.php).
Now I ran a duplicate content checker called Siteliner (www.siteliner.com) for the first time and it listed both the tidied and the complete urls, marking them as duplicate content.
Is that a misunderstanding inside Siteliner or is that a real issue in SEO?

That would be most likely a bug in the siteliner product. A “tidy” link does not do anything as far as adding content. It simply removes the index.php or index.html from internal page links and lets the server use default values.

If you don’t give a full file name to a web server it will look for index.html and index.php in the directory, if neither are found you will get a 404 not found.

Of course this is the default for most web servers that can be changed at the server configuration settings.