When a person performs an act that contains two forbidden actions, is he held responsible for both, or only for one? If, for example, someone who is not a kohen enters the Temple on Shabbat and performs the Temple service, is he held liable only for performing the Temple service (which is forbidden to someone who is not a kohen – see 18:7), or also for the act of sacrificing on?
This basic situation is referred to by the Gemara as issur hal al issur, and we find an argument between Rabbi Hiyya and Bar Kappara on this matter, with Rabbi Hiyya ruling that the person would be held liable for both, and Bar Kappara ruling that they would only be responsible for one sinful act. Both Rabbi Hiyya and Bar Kappara are adamant about their positions – the Gemara quotes each of them as swearing that their ruling was the position taught by Rabbi Yehuda HaNasi.
The Gemara in Shevu’ot (26b) discusses a case like this one and concludes that neither person would be held responsible for taking a false oath, since they were each certain of their positions. Rabbi Shmuel HaLevi, in his Ramat Shmuel, adds that in this case it is even possible that Rabbi Yehuda HaNasi changed his mind at some point and taught this law differently at different times, so it is plausible that both of his students were telling the truth.
Bar Kappara was one of the last tanna’im, a student of Rabbi Yehuda HaNasi (Rebbe) and a friend of Rabbi Hiyya. We are never told his first name, although some suggest that his father was Rabbi Elazar HaKappar, who died before he was born, and that he, too, was named Elazar. He was knowledgeable in both Torah (he authored a collection of baraitot known as Mishnat bar Kappara) and in general knowledge, which is why he was sent on several occasions as the Jewish emissary to the Roman government. Almost all of the first-generation amora’im in Israel were his students. He was known as a satirist with a healthy sense of humor, and even offered critique that extended to Rabbi Yehuda HaNasi and his family, which may explain why – his close relationship with Rebbe notwithstanding – he did not receive Rabbinic ordination until after Rebbe’s death.