[Discuss] fdupes opposite?

[Murray Strome]

Does anyone know of a utility that does the opposite of fdupes?

fdupes - finds duplicate files in a given set of directories

This has several options, including searching through subdirectories. It 
uses md5sum and finds duplicates even though the names may be different 
(see man fdupes for details).

I am looking for a utility that would be similar, except that instead of 
finding and listing those files which are duplicated in a directory 
tree, it would find those that are unique and list those.

Does anyone know of a utility that does this?

Thanks.

Murray