Don't double count hard link files.

BUG: 406030 On posix file systems, if the number of links for a file is >1 add it to a set the first time we count it and never count it again. Fixes double counting files that aren't taking twice as much space.

Seems to work fine here. I had it scan a folder with a couple of hard links, only the first one was added. But it also only showed the first filename it saw of course in the gui.