dired-find-duplicates protects against . and ..

Don't be a jerk and mark . and .. for checking. They will get expanded
recursively and it'll never end.
dired-find-duplicates handles missing *duplicated files* buffer.
dired-find-duplicates uses insert-file-contents-literally

insert-file-literally is the wrong function to call.
dired-find-duplicates fix buffer killing.
dired-find-duplicates fixes file hierarchy recursion