100:19 If a kohein already blessed the people and he later went to another shul, he can go up and duchen there as well if he so desires. If he doesn’t want to, even though he heard them call out “kohanim,” he doesn’t have to go up since he has already done so once. (If he does duchen a second time, he repeats the bracha on the mitzvah, even in the same shul – Mishnah Brurah 128:11.)
100:20 If a kohein has a blemish on his face, he may duchen in our countries, where the practice is for a kohein to fold his tallis over his face. However, if a kohein has a blemish on his hands, such as if they have white spots or small dots, or if they are bent or he can’t separate his fingers, then he may not duchen. This is because the congregation will stare at this and be distracted. (In a place where the kohanim also cover their hands with the tallis, he may duchen – MB 128:117.) If he’s known in this town, meaning that has been there thirty days so that they recognize him, he may duchen. If the kohein’s hands are stained from his work, he doesn’t duchen because the congregation will be staring at them but if most people in the city work in such a capacity, he may. A kohein who cannot pronounce the letters correctly, such as one who pronounces the letter “shin” as if it were the letter “sin,” may not duchen unless the congregation also pronounces the letter in this way. Any kohein who doesn’t duchen should leave the shul before “R’tzei” until they have concluded.