The Shulchan Aruch (128) writes that even though a Kohen washed his hands in the morning before Shacharit, he must wash his hands again before reciting Birkat Kohanim (the priestly blessing). This ruling is in opposition to the view of the Rambam (Spain-Egypt, 1135-1204), who held that if a Kohen was careful to keep his hands clean since the morning washing, he need not wash them again before Birkat Kohanim.
Some Egyptian Jewish communities have the practice of following the Rambam's position, and do not require Kohanim to wash their hands again before Birkat Kohanim. It should be noted, however, that the Rambam's leniency applies only to the Birkat Kohanim of Shacharit. Before Birkat Kohanim of Musaf, however, a Kohen must, according to all opinions, wash his hands even if he had washed them earlier that morning. Since a long period of time elapses before Musaf, a Kohen may not rely on his earlier washing, and must wash his hands again. Therefore, even in those communities who follow the Rambam's ruling, where the Kohanim do not wash again before Shacharit, the Kohanim must wash their hands in preparation for the Birkat Kohanim of Musaf.
Other communities follow the ruling of the Shulchan Aruch and require that Kohanim wash their hands again for Birkat Kohanim even during Shacharit. However, Chacham Ovadia Yosef, in Halichot Olam (Parashat Tetzaveh, 3), discusses three situations in which Kohanim may rely on the Rambam's view. The first is when only ten men are present in the synagogue, and the Minyan would thus be lost if the Kohen steps out to wash his hands. In such a case, the Kohen should preferably rely on his washing earlier that morning, in accordance with the Rambam's position, rather than break the Minyan. Secondly, if the Kohen must pass in front of somebody praying the Amida in order to leave the synagogue and wash his hands, he should rely on his earlier washing, rather than disrupt that person's concentration by walking in front of him. Finally, if the Chazan (person who leads the services) is a Kohen and will recite Birkat Kohanim, and he did not wash his hands before beginning the repetition of the Amida – which is the preferred practice – he may rely on his earlier washing and recite Birkat Kohanim without washing his hands a second time. Ideally, however, a Kohen who leads the services should wash his hands in preparation for Birkat Kohanim before beginning the repetition of the Amida.
It must be emphasized that this applies only to Birkat Kohanim of Shacharit; in Musaf, a Kohen may not rely on his earlier washing, even in these three situations.
Summary: Some Egyptians communities do not require a Kohen who had washed his hands before Shacharit to wash again before Birkat Kohanim in Shacharit. Even according to this practice, a Kohen must wash his hands again before Birkat Kohanim of Musaf. Other communities do not allow a Kohen to rely on his earlier washing even for Birkat Kohanim of Shacharit, except in three cases: if he would break the Minyan by leaving the synagogue to wash; if he would have to pass in front of somebody praying the Amida; or if he is the Chazan and he forgot to wash his hands before beginning the repetition of the Amida.
