The Shulhan Aruch (Orah Haim 128; listen to audio recording for precise citation) rules that if a Kohen finishes the morning prayers in one synagogue and later happens to find himself in another synagogue, he may participate in Birkat Kohanim a second time. Even though he had already recited Birkat Kohanim at the Minyan where he prayed, he may recite the blessing again in the second synagogue. The Mishna Berura (commentary by Rabbi Yisrael Meir Kagan, 1839-1933) explains that performing a Misva a second time when a new opportunity arises does not violate the prohibition of "Bal Tosif," adding onto the Torah’s commands (listen to audio recording for precise citation).
The Mishna Berura also clarifies that the Kohen is not obligated to perform Birkat Kohanim in the second synagogue, since he had already performed the Misva once that day. His participation in Birkat Kohanim a second time is optional. Nevertheless, he may recite the Beracha over the Misva ("Baruch Ata…Asher Kideshanu Bi’kdushato Shel Aharon Ve’sivanu Le’barech…") when he joins in Birkat Kohanim at the second Minyan. Even though the Beracha states that God "commanded us" to administer Birkat Kohanim, it may be recited in this situation, when Birkat Kohanim is optional.
The Shulhan Aruch (128:29) addresses the situation of a Kohen who arrives late in the synagogue in the morning, when the Kohanim are preparing for Birkat Kohanim. Under normal circumstances, as the Shulhan Aruch rules, the Kohen should join the other Kohanim for Birkat Kohanim, and then proceed to pray Shaharit. The Mishna Berura addresses the more complex case where the Kohen will miss the final time for Shaharit if he participates in Birkat Kohanim before praying Shaharit. In such a case, the Mishna Berura writes, the Kohen should go outside the sanctuary and pray the Amida, rather than recite Birkat Kohanim and miss the final time for the Shaharit prayer. However, if he is in the synagogue when the Kohanim are called up to recite the Beracha ("Kohanim Be’chabod"), then he bears a Torah obligation to go and recite Birkat Kohanim. Since the morning prayer service is required on the level of Rabbinic enactment, and not on the level of Biblical obligation, Birkat Kohanim takes precedence over reciting Shaharit before the final time.
The Mishna Berura also discusses the case of a Kohen who arrives in the synagogue at the time of Birkat Kohanim, and if he participates in Birkat Kohanim he will miss the final time for reciting the Shema. In such a case, the Mishna Berura writes, the Kohen should quickly recite the verse of "Shema Yisrael Hashem Elokenu Hashem Ehad" as well as "Baruch Shem Kebod Malchuto Le’olam Va’ed," and then participate in Birkat Kohanim. As the basic requirement of Shema constitutes a Biblical obligation, the Kohen should not miss the final time for Shema in order to participate in Birkat Kohanim. He should instead recite the first verse, and "Baruch Shem," and then join the Kohanim for Birkat Kohanim.
These Halachot demonstrate the unique importance that Halacha affords to the Misva of Birkat Kohanim, which, in certain circumstances, is even given precedence over the morning Amida prayer.
Summary: A Kohen who had already performed Birkat Kohanim happens to later be in another Minyan for Birkat Kohanim may, if he wishes, participate in Birkat Kohanim, with the Beracha. If a Kohen arrives late in the synagogue, at the time when the Kohanim are preparing for Birkat Kohanim, then he should join them and then recite Shaharit. If he will miss the final time for Shaharit if he participates in Birkat Kohanim, then he should leave the synagogue and pray the Amida. If, however, he was in the synagogue when the Kohanim were summoned for Birkat Kohanim, then he should join in Birkat Kohanim, even if he will thus be unable to recite the Amida before the final time. If by participating in Birkat Kohanim he will miss the final time for Shema, then he should quickly recite the verse of "Shema Yisrael" and "Baruch Shem" and then participate in Birkat Kohanim.
