At first glance the whole question of whether a non-kohein, a zar, can duchen, as absurd. The command of birchas kohanim is directed to bnei Aharon, and the gemara in Kesubos(24) indicates that this constitutes an issur aseh for all others. Yet, Chazal tell us that R' Yosi, who was not a kohein, praised himself for always obeying his friends' advice (one can only speculate how R' Yosi would have replied to the favorite question of mother's everywhere: would you listen to your friends if they told you to jump off a bridge?) even to the point of going up to duchen at their behest(Shabbos 118b). Tosfos quotes R"I who was perplexed at the praise - there is no issur for a zar to duchen, writes Tosfos, so why should R' Yosi have not been willing to fulfill his friends' request, strange as it may have been? How did the R"I deal with the explicit issur aseh menationed by the gemara in Kesubos?
The simplest answer for Tosfos is that the gemara in Kesubos meant that there is an issur for a zar to recite birchas kohanim, but for a zar to simply stand on the duchen with the kohanim while they recite the bracha is not a problem (see Aruch haShulchan siman 138, Shu"t Oneg Y"T O.C. #15 who also has an interesting discussion of the MG"A who attempts to hinge the issur aseh on a machlokes tana'im found elsewhere). The Minchas Chinuch suggests that the gemara in Kesubos might be speaking only of birchas kohanim in the mikdash where shem hemeforush was used (which suggests that the issur aseh is not particular to birchas kohanim, but is a violation of pronouncing Hashem's name unneccesarily - see maharasha shabbos 118).
These answers are so good they force us to redress Tosfos' original question - if indeed there is no issur of being oleh l'duchan, what is the big deal of R' Yosi listening to his friends when they told him to do so? Perhaps one can bring proof from here to the position of the sefer chareidim (cited by minchas chinuch) that there is a mitzvah on a yisrael to be blessed by the kohanim. By being oleh to duchen, R' Yosi was forfeiting this kiyum mitzvah for the sake of the middah tova of listening to his friends. The Meshech Chochama suggests a similar approach: the gemara (Sota 38) tells us that workers in the fields who are onusim, pervented by circumstance, from coming to hear birchas kohanim, are included in the bracha, but those who can come to listen but intentionally avoid doing so are not included. By being oleh to the duchen R' Yosi was forfeiting the opportunity to receive birchas kohanim. However, the gemara is mechadesh that since R' Yosi always followed the midah tovah of listening to his friends, he was considered onus and still included in the bracha.