TT #107 · Parshas Tazria · Aliyah 2 · Passuk 10
English: The kohen shall look [at it]. And, behold! there is a white se'eith on the skin, and either it has turned the hair white, or there is healthy, live flesh in the se'eith.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:10
Torah Temimah cites Sifra (Toras Kohanim) for the requirement to examine an additional lesion that appears after the priest has been summoned.
Sifra — Vayikra 13, Midrash Sefer
English: If he was called to see one lesion and another developed, that lesion too requires examination.
Textual derivation
The priest’s summons is not limited to the first mark: if another lesion appears, it must also be examined.