I'm having this same difficult time deleting a lot of duplicate files. One idea I have (now that I've backed everything up) is to check the containing folders and delete entire folders. This will save me from clicking thousands of files.
I hope that is a good suggestion.
This is also my question, but where do I find Categories? I have over 70% listed as "other files."
Also, thanks for your signature, I just realized I've never backed up this computer!
Thanks!