Commentaire sur Le Lévitique 13:9

A skin-eruption of tzora’as. Although it has previously been clarified that the word nega (skin-eruption) is only used when it is already [known to be] unclean, nevertheless, this usage of the word here comes to teach us that even if the man knows by himself that in the end it will be unclean he must still be brought to the kohein.