Berachos, Chapter 5, Mishnah 4. The Mishnah deals with the shaliach tzibbur, the "chaver" who stands before the ark and leads the prayers, and with how he should conduct himself while the kohanim go up to the duchan.
"He should not answer amen after the kohanim":
"Chaver lifnei ha-teivah lo ya'aneh achar ha-kohanim amen" - when the kohanim go up to the duchan and recite Birkas Kohanim, the shaliach tzibbur should not answer amen after the blessings they say ("Yevarechecha Hashem ve-yishmerecha"). And the reason is: "mipnei ha-tiruf" - out of concern that he will lose his concentration, and once he has answered amen he will be unable to call out the next blessing to the kohanim, or to continue with the Shemoneh Esrei that follows.
The berachah the kohanim recite at the outset:
And what is the law regarding the berachah that the kohanim themselves recite before Birkas Kohanim - "asher kideshanu bi-kedushaso shel Aharon ve-tzivanu levarech es amo Yisrael be-ahavah"? Some of the poskim (the Rema among them) rule stringently that the shaliach tzibbur should not answer amen after it, because this amen constitutes an interruption in the prayer. The amen answered to Birkas Kohanim itself is different, since that is considered part of the prayer, and therefore the only reason there is the concern of losing concentration.
A shaliach tzibbur who is the only kohen:
"Ve-im ein sham kohen ela hu - lo yisa es kapav" - a shaliach tzibbur who is the only kohen present should not go up and raise his hands to bless the people, out of concern that after the blessing his mind will not remain clear and he will not know how to return to the continuation of the repetition of the Shemoneh Esrei.
"Ve-im havtachaso she-hu nosei es kapav ve-chozer li-tefilaso - rashai" - if he is confident in himself that he will be able to raise his hands in blessing and then return to his prayer, he is permitted to do so.
And some say that this permission, even for one who is confident in himself, applies only when there is no other kohen present. But if there is another kohen there, he should not raise his hands in blessing, even though he is confident in himself.
In summary: In this Mishnah we learned three halachos concerning the conduct of the shaliach tzibbur during Birkas Kohanim: that he should not answer amen after the kohanim, because of the concern that he will lose his concentration; that some rule stringently that he should not answer amen even after the berachah the kohanim recite, because it is an interruption in the prayer; and that a kohen standing before the ark should not raise his hands in blessing unless he is confident that he will be able to return to his prayer - and according to the view of "some say," even that only when there is no other kohen present.