Duplicate png files

Hi All . . .
I was looking through stuff on my Mac mini and found a bunch of .png files with duplicate pictures. I cannot remember how I got to where they were or how they were duplicated...or when it happened. How can I once again find and delete all duplicate files throughout my computer? I have done a forum search on this. How do files like .png files get duplicated? What do I do in the future to prevent this? Thanks!
And another thing...I use Skype and found an application called Skype_old...what has happened there?
I do not have iPhoto and most of the duplicated .png files are the ones that come with the OS.

Have you updated Skype recently. When I upgraded the version of Skype that was on my system got renamed to Skype_old. If the new version is working well for you it's ok to delete Skype_old.