Adding page name twice to url works

All,
I have created web application and hosted on IIS. Everything works fine with no issues. Lets say the url is - http://mydomain/folder/test.aspx, the page shows up fine. But i also see the page if i type the url like this - http://mydomain/folder/test.aspx/test.aspx
I am not sure how does this work. Has anyone experienced this? How to not load the later url, but show a custom error page or some message instead 

thanks!