TT #305 · Parshas Tazria · Aliyah 7 · Passuk 58
English: The garment, warp or woof, or any leather article that you wash, if the lesion disappears from it, shall be washed a second time and be clean.
Torah Temimah cites Sifra (Toras Kohanim) for distinguishing the first washing, which initiates quarantine, from the second, which purifies.
English: The second washing is for purification; the first washing places the lesion under quarantine.
Textual derivation
The first washing initiates the isolation period; the second washing purifies the article when the conditions are met.