The main difference between paid and free duplicate file finders is that the free programs simply look for identical file names. Some of the paid programs actually check each file's contents to see whether there is a match with another file. Useful if you have the same photo stored under several different names.
There is a slight danger when casually deleting duplicate files that you can accidently delete files which are duplicates but need to be in two separate directories on the computer because a couple of programs need them to be in specific directories for that program to work. This probably only applies to older programs, but something to be aware of before emptying the recycle bin.