The Obligation of a Kohen to Perform the Priestly Blessing a Second Time in a Day
Question
Answer
He is not obligated to ascend to the platform again, but if he does, he fulfills a mitzvah and recites the blessing "Who has sanctified us." If he was told to ascend (before "Retze"), he has a rabbinic obligation.
Source
Gemara, Rosh Hashanah, page 28: "Since another congregation came his way - he blesses again, the whole day is his time."
This is also ruled in Shulchan Aruch, Orach Chaim, Siman 128, paragraph 28: "A Kohen who raised his hands, and then went to another synagogue and found a congregation that had not yet reached the priestly blessing, can raise his hands again." Mishnah Berurah there, s.k. 106, notes that the Shulchan Aruch carefully chose the word "can," since after he has already ascended, he has no obligation to ascend again. Nevertheless, he blesses each time as they bless in Shacharit and Musaf, and no one objects.
Nevertheless, if he was told to ascend before "Retze," Pri Megadim in A.A., s.k. 3, writes that he has a rabbinic obligation, even if he has already ascended.