du: ignore duplicate paths provided by user

[cakebaker]

This PR ignores duplicate paths provided by the user and so du a a a has the same output as du a. However, if the path doesn't exist, multiple error messages are shown. This matches the behavior of GNU du.

[github-actions[bot]]

GNU testsuite comparison:

Congrats! The gnu test tests/tail/retry is no longer failing!

[cakebaker]

Good catch, thanks!

Yes, I agree, it makes sense to get the whole thing done.

[sylvestre]

@cakebaker are you planning to finish it? thanks

[BenWiederhake]

Note that all of this could be just thought of as weird hardlinks, because hardlinks have to be detected anyway:

$ ln README.md README.md.hd
$ ../gnu/src/du -bc README.md README.md.hd # It's a hardlink, don't count it twice
7973	README.md
7973	total
$ cargo run du -bc README.md README.md.hd # Whoops!
7973	README.md
7973	README.md.hd
15946	total