TT #119 · Parshas Tazria · Aliyah 2 · Passuk 10
English: It is chronic leprosy on the skin of his flesh, and the priest shall pronounce him impure; he shall not isolate him, for he is impure.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:11
Torah Temimah cites Sifra (Toras Kohanim) for the rule against a new isolation period for a person already declared definitively impure.
Sifra — Vayikra 13, Midrash Sefer
English: “He shall not shut him up”: this teaches that one who has already been declared definitively impure is not confined for another observation period.
Textual derivation
Once the priest has declared the person definitively impure, the ordinary observation-period procedure is not restarted.