Rashi on Deuteronomy 18:6/רש"י על דברים י״ח:ו׳

1וכי יבא הלוי AND IF A LEVITE COME […AND MINISTER] — One might think that Scripture speaks of a “Levite” in the usual sense of the word (i.e. of a לוי and not of a כהן)! Scripture, however, goes on to state, (v. 7) “And he shall minister [in the name of the Lord]”; thus the Levites must be excluded for they are not fit for (not entitled to) service in the Temple (Sifrei Devarim 168:1).

2ובא בכל אות נפשו ושרת AND COME WITH ALL THE LONGING OF HIS SOUL …AND MINISTER — This teaches that a priest may come and offer his free-will and obligatory sacrifices even at a time when a priestly shift is in charge to which he does not belong (Bava Kamma 109b). Another explanation: It further teaches regarding priests who appear in the Temple as pilgrims on the festivals that they may offer [with the rota], and do the services connected with the sacrifices that are offered on account of the festival, as for instance, the additional offerings due on festivals — although the shift is not theirs (Sifrei Devarim 168:3; Sukkah 55b).