Parashas Tetzaveh · Passuk 28:40
Chabad.org — English translation + Rashi
Torah Temimah points to Yerushalmi Yoma 3:6.
English: The Sages derive from the plural “kutonos” that two tunics should be prepared for each priest, so that a replacement is ready when needed. The requirement concerns having a spare garment available; it does not mean wearing two tunics at once.
In context: See the cited source for the full section.
Derash on the verse.
The plural form of “kutonos” is understood as requiring spare tunics for the priests.