Gemara

(Beraisa): "Daber El Bnei Yisrael... v'Somach" - Bnei Yisrael do Semichah (press on the neck of a Korban), but Benos Yisrael do not;

R. Yosi and R. Shimon say, Benos Yisrael may do Semichah if they want.

A Stam Sifra (an anonymous Beraisa that expounds a verse in Vayikra) is R. Yehudah.

Shabbos 118b (R. Yosi): I never transgressed the words of my colleagues. I know that I am not a Kohen. Even so, if they would tell me go to the Duchan (where Kohanim give Birkas Kohanim), I would go!

Kesuvos 24b - Question: If we see a man Duchan, does this establish him to have proper lineage?

For a Zar to Duchan is only an Isur [inferred from an] Aseh.

Rishonim

Tosfos ha'Rosh (118b DH Ilu): R. Yosi would not truly Duchan. An Isur Aseh forbids this! He means that he would do a great matter for his colleagues.

Rashi (24b DH v'Isur): The Isur for a Zar to Duchan is "Ko Sevarchu" - you (Kohanim), and not Zarim. A Lav inferred from an Aseh is an [Isur] Aseh.

Tosfos (118b DH Ilu): The Ri did not know what is the Isur for a Zar to Duchan, if not for a Berachah l'Vatalah. The Torah told the Kohanim to bless Yisrael.

Poskim

Rema (OC 128:1): A Zar may not Duchan even with other Kohanim. (Kesuvos 24b says that he transgresses an Aseh. The Ri did not know what is the Isur.)

Bach (1 DH Garsinan): It is an Isur Aseh for a Zar to Duchan (Kesuvos 24b). Why did R. Yosi say that if his colleagues would tell him to go to the Duchan, he would go? He connotes that he would bless! He cannot refer to the Duchan where the Leviyim sang. He said "I am not a Kohen"! Also, he was after the Churban! The Ri understood that he would bless. How could he transgress an Isur Aseh to obey his colleagues?! Even if he would not bless "Asher Kidshanu bi'Kedushaso Shel Aharon...", he transgresses the Aseh "Ko Sevarchu" (Kohanim will bless), and not Zarim! Surely, Berachos [before Mitzvos] are not Me'akev. Even if one's father tells him "cook for me on Shabbos", one may not obey, for "Ani Hash-m" - all of you must honor Me (Bava Metzi'a 32a). All the more so one may not obey his colleagues to transgress an Isur Aseh. Why didn't the Ri know what is the Isur?

Magen Avraham (1): The Darchei Moshe answers for the Ri, that in Kesuvos the Zar is alone. R. Yosi discusses with Kohanim. What is his source? Perhaps the Ri explains that the Isur Aseh is saying Hash-m's name l'Vatalah in a Berachah. I say that R. Yosi would ascend to the Duchan and bear disgrace, but not bless. See the Maharsha (who says so). Alternatively, R. Yosi holds that "Daber El Bnei Yisrael" excludes women from Semichah, but it is Reshus for them. Likewise, it is Reshus for a Zar to Duchan. Kesuvos 24b calls it an Isur Aseh. That is like R. Yehudah, who forbids women to do Semichah. This is primary.

Gra (DH v'Ein) concludes "see Magen Avraham" (he agrees with him).

Noda bi'Yehudah (1 OC 6): R. Shimon lists 16 Avodos given to Kohanim (Menachos 18b). The Mishnah disagrees, for it is Machshir Blilah (mixing the flour with the oil) of a Zar. One of the 16 is Birkas Kohanim. This does not prove that a Zar may not Duchan. It says about the Avodos "Bnei Aharon ha'Kohanim", but we needed another source that a Zar disqualifies the Avodah, and "v'Zar Lo Yikrav" to forbid a Zar. Perhaps Birkas Kohanim is Reshus, like the Magen Avraham says! Perhaps "v'Zar Lo Yikrav" forbids a Zar to Duchan only in the Mikdash, where "El ha'Mizbe'ach Lo Yikrevu" applies. (Hagahah - perhaps outside the Mikdash is like a Bamah, where there is no Isur Zarus.) However, Tosfos (Zevachim 16a DH Heichan) learns an Isur from "v'Hikrivu Benei Aharon", and a Lav from v'Zar Lo Yikrav. If it is Reshus for a Zar to Duchan, we could say similarly about [Avodah, and not learn an Isur from] v'Hikrivu Benei Aharon!

Chasam Sofer (Kesuvos 24b DH Nesi'as): The Rambam and others forbid a Berachah l'Vatalah due to the Lav "Lo Sisa [Es Shem Hash-m Elokecha la'Shav]". Tosfos (Rosh Hashanah 33a) holds that it is only mid'Rabanan. No one calls it an Aseh! I say that there is no Torah Isur for a [true] Berachah l'Vatalah, e.g. he praises Hash-m for a food but does not eat it now, or "Mekadesh ha'Shabbos" on a weekday, or a woman says "Asher Kidshanu..." regarding Sukah (Hash-m commanded His nation). If a Zar says Asher Kidshanu bi'Kedushaso Shel Aharon, this is false. It is not Shem Hash-m la'Shav. Even saying only Baruch Atah Hash-m is not in vain [for he blessed Hash-m]. However, he transgresses the Aseh to fear Hash-m, for he uses Hash-m's name for Sheker. R. Yosi would not transgress Torah law to obey his colleagues. He merely casts off fear of Hash-m, and lies about himself, without intent to deceive. He intends to honor Chachamim. The Torah taught that we must fear Chachamim along with fearing Hash-m! Therefore, there is no Isur.

Note: If so, why don't we bless in any case of Safek? This is not casting off fear of Hash-m!

Chasam Sofer (DH Od): R. Yehudah forbids women to do Semichah. It is use of Kodshim without a Mitzvah to permit it. When there is no Isur, all permit doing a Mitzvah from which one is exempt. The argument about women doing Semichah does not apply to Birkas Kohanim! (We must say that Bal Tosif does not apply.) Rather, R. Yosi learns from "Daber El Aharon" that Zarim are not obligated. Ko Sevarchu teaches that a Zar has no Mitzvah at all, but there is no Isur. R. Yehudah learns from "Daber El Aharon" that Zarim have no Mitzvah, so Ko Sevarchu forbids them.

Note: Through obeying his colleagues and Duchaning, he transgresses the opinion of his colleague R. Yehudah!

Sha'arei Teshuvah (2): Teshuvos in Sefer Hafla'ah says in the name of Sefer Charedim that there is an Aseh for a Yisrael to receive Birkas Kohanim. A Zar who ascends the Duchan is not blessed, like we say about Yisre'elim in back of the Kohanim.

Mishnah Berurah (3): The Pri Megadim connotes that there is an Isur Aseh to bless even without raised hands, unlike the Bach. Magen Giborim says that the Isur Aseh is only if he intends for the Mitzvah, but not if he merely accedes to his friends who think that he is a Kohen. I say that this is only because we hold that Mitzvos require intent. If not, he must have special intent not for the Mitzvah.

Bi'ur Halachah (DH v'Ein): The Pri Megadim agreed with the Pri Chodosh that a Chalal (a Pasul Kohen) transgresses just like a Zar. Even if he ascended, he must descend.

Bi'ur Halachah (DH d'Zar): Why do people, even Zarim, bless Yevarechecha... when accompanying friends? The Yerushalmi says that Birkas Kohanim is only at the time of Tefilah, i.e. the enactment is only then. Mid'Oraisa it does not depend on Tefilah. Most Poskim say that Tefilah is only mid'Rabanan! The custom for Zarim to bless this way shows that Mitzvos require intent. However, perhaps people hold like the Bach, who forbids only with raised hands. Alternatively, since Chachamim enacted not to Duchan without Tefilah, it is as if a Kohen or Zar who says these verses at any other time explicitly intends not for the Mitzvah.

Kaf ha'Chayim (7): Shevus Yakov (1:93) connotes that a Safek Kohen Duchans. Pesach ha'Dvir and Mishbetzos Zahav (27) disagree.

Rema (ibid.): Perhaps it is permitted with other Kohanim. This requires investigation.

Bach (DH umi'Divrei): The Rema connotes that the Isur Aseh is only when the Zar is alone. When there are Kohanim with him, the Ri did not understand the Isur. This is astounding. It seems that the verse forbids in every case, even if he does not bless "Asher Kidshanu..."! Rather, R. Yosi meant only that he would go to the Duchan, but not raise his hands or bless "Yevarechecha..." There would merely be slander that he transgressed an Aseh. He preferred to bear that than to transgress his colleagues' words. However, Tosfos understood differently. Rather, he would bless without raised hands and without Asher Kidshanu. The Ri holds that the Isur is only with raised hands, like it says Ko Sevarchu (so you will bless), like it says (Vayikra 9:22) "va'Yisa Aharon Es Yadav va'Yevarechem". Tosfos holds that there is no concern for slander, since it is known that he blesses without raised hands.

Taz (2): "Even with other Kohanim" is not precise. The Rema comes to answer Tosfos' question. The Isur Aseh is only if he blesses alone, without Kohanim. I disagree. Rather, Tosfos holds that a Kohen has a Mitzvah, but a Zar has no Isur. We find (Yevamos 54b) 'perhaps "Yevamah Yavo Aleha" is a Mitzvah after her husband dies, and in his lifetime it is optional! Or, perhaps she is forbidden in his lifetime due to an Aseh.' This shows that we need not expound an Isur Aseh when we could expound that sometimes it is a Mitzvah, and sometimes it is optional. Also here, perhaps there is no Isur Aseh. This is not Avodah! Tosfos concludes "the Torah commanded Kohanim", i.e. the command is [only] to them. I say that R. Yosi meant that if his colleagues told him that he is a Kohen, he would nullify his opinion and accept theirs. We find that even though Beis Hillel and Beis Shamai argued, they did not refrain from intermarrying (Yevamos 13b).

Levushei Serad (1): The Gemara said that this is because they informed each other if there was a problem according to the other group! The Taz learns from the Hava Amina, that they argued l'Halachah, but not in practice.

Noda bi'Yehudah (ibid.): The Magen Avraham answered "perhaps the Ri explains that the Isur Aseh is saying Hash-m's name l'Vatalah in a Berachah." I.e. he must bless Asher Kidshanu if he Duchans alone. With others, he can hear this Berachah from them. The Ri did not know what is the Isur, if not for a Berachah l'Vatalah, and R. Yosi would not do so (Duchan alone)! If the Magen Avraham's concern were for Hash-m's name in Yevarechecha..., this is even with other Kohanim. However, the Magen Avraham rejected this distinction. I lean to say that his concern is for "Hash-m's name l'Vatalah" in Yevarechecha... He said "see Sof Siman 215." (He said there that the Isur of an unnecessary Berachah is only mid'Rabanan.)

Chasam Sofer (DH Emnam): A Zar has an Isur Aseh only when a Kohen has an Aseh, i.e. when he is called to Duchan. The Shali'ach Tzibur intends to call only Kosher Kohanim! The Isur Aseh is if the Shali'ach Tzibur intentionally calls Zarim to Duchan when there are no Kohanim. In Kesuvos we say that there is an Isur Aseh because the Shali'ach Tzibur thinks that he is a Kohen. The Rema means that there is no Isur if he Duchans with Kohanim, and the Shali'ach Tzibur does not intend for him.

Chasam Sofer (DH v'Od): R. Peretz (in Tur 128) says that "Amor Lahem" is only when there are at least two Kohanim. R. Yosi holds that a lone Kohen has no obligation, but a Mitzvah of Reshus, just like Semichah is Mitzvas Reshus for women. A lone Yisrael has an Isur Aseh. R. Yehudah holds that women have no Mitzvah of Semichah, and a lone Kohen has no Mitzvah, so a lone Yisrael has no Isur. R. Yosi said 'I hold that I may not Duchan, but if my colleague (R. Yehudah), who permits, would tell me, I would obey, against my opinion.

Mishnah Berurah (6): The Acharonim conclude that in every case he transgresses the Aseh.