TT #95 · Parshas Tazria · Aliyah 1 · Passuk 5
English: On the seventh day the priest shall conduct an examination, and if the affection has remained unchanged in color and the disease has not spread on the skin, the priest shall isolate them for another seven days.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:5
Torah Temimah cites Sifra (Toras Kohanim) for the count of the second seven-day period.
Sifra — Vayikra 13, Midrash Sefer
English: “Seven days a second time” teaches that the seventh day is included in the count, whether counted from before or after it.
Counting rule
The seventh day is included in the reckoning of the second seven-day period, with the count understood relative to the prescribed period.