100:17 One should try to arrange things so that the shliach tzibbur is not a kohein. If he is a kohein, he does not go up to duchen. Also, he should not read the words to prompt the kohanim. In such a case, another person stands next to him to call out “kohanim” and to prompt with the words “Yivarechecha,” etc. The shliach tzibbur stands silently until it is time for him to recite “Sim Shalom.” (If there is no one else present who knows how to read the prompts, then the kohein shliach tzibbur may do so – Mishnah Brurah 128:87.) He does not transgress by not going up to duchen, even though the other person called the kohanim to go up, because the intention is only to call those who are not in the middle of prayer and who left their places at “R’tzei.”
If there is no kohein in shul other than the shliach tzibbur, then, so that the mitzvah of blessing the people should not be done away with, he should go up to duchen, relying on the morning hand-washing. The procedure is then as follows: the kohein moves a little from his place (to go towards the platform – MB 128:77) when he says “R’tzei;” he continues through the end of the bracha, after which he says “Elokeinu v’Elokei avoseinu…” The shliach tzibbur then goes up to duchen while the other person reads the prompts. The shliach tzibbur then returns and says “Sim Shalom.” He doesn’t say the prayer “Ribon haolamim…” until after kaddish. If the kohein did not move from his place at “R’tzei,” he is not permitted to go up.
100:18 If a congregation is made up entirely of kohanim, if there are only ten present, they all go up to duchen (except for the shliach tzibbur – MB 128:97). Who are they blessing? Their fellow Jews who are in the fields. Who responds Amen? The women and children who are in shul. Even if there are no women or children present, the response of Amen is not an indispensible component. If there are more than ten kohanim present, those over the ten go up to duchen while ten stay in their places to respond Amen.