Finding & removing redundant files

Not sure where my question belongs. Hope this fits in here.

My ext. HDD is pretty cluttered with potentially redundant files (due to a pretty stupid backup strategy -> "copy&paste + several different folders and not remembering where I put what").

Finding them the search function is kind of a drag, so I wanted to ask you guys if there's a program that lists all redundant files, so I can easily find and deal with them?

I'm using Win8.1 64bit.

I have used something called 'duplicate cleaner free', which would found all the double up files which could then be deleted from with in the softwear, maybe give that a shot

Thx, I'll check it out.