Gemara

(Mishnah): If a Nazir was Mitamei (exposed himself to Tum'as Mes) all day, he is liable only once. If they warned him 'don't be Mitamei, don't be Mitamei' and he was Mitamei, he is liable for each warning.

(Rabah citing Rav Huna): The Torah says "he will not become Tamei', and also 'he will not enter (Ohel ha'Mes, a tent in which there is a Mes). It discusses all situations (even if he is already Tamei, he is liable for entering Ohel ha'Mes);

He is not lashed twice for Tum'ah twice if the latter Tum'ah is not via an Ohel.

Rav Yosef: I swear, Rav Huna said that even for touching twice, he is liable twice!

(Rav Huna): If a Nazir was in a cemetery and they passed his (dead) relative or any Mes to him, and he touched it, he is liable.

Question (Abaye - Beraisa) Suggestion: If a Kohen was carrying a Mes and they passed his relative or any Mes to him, and he touched it, perhaps he is liable!

Rejection: It says "He will not profane" (most change the text to "to profane himself"). The Torah does not discuss one who is already profaned (Tamei).

Counter-question (Rav Yosef - Mishnah): If they warned him 'don't be Mitamei, don't be Mitamei' and he was Mitamei, he is liable for each warning.

He is already Tamei, yet he is liable for touching Tum'ah, unlike the Beraisa!

Answer: In the Beraisa, the Tum'os are connected. In the Mishnah, they are not.

Question: If connected Tum'ah were mid'Oraisa, it would apply in all cases, but it does not!

(Rav Yitzchak bar Yosef): Connected Tum'ah applies only to Terumah and Kodshim. It does not apply to a Nazir or to one who offers Korban Pesach.

Answer: Rav Yitzchak discusses connection to Tum'ah through people (touching one who touches a Mes.) The Beraisa discusses connection to the Mes (touching a second Mes before separating from the first).

Question: Surely, Abaye exempts because he is already Tamei. The same applies when a Tamei Kohen enters Ohel ha'Mes!

Answer #1 (Rav Papa): The case is, he entered the Ohel in a box, and his friend uncovered the box. He becomes Tamei at the same moment he enters.

Answer #2 (Mar bar Rav Ashi): The case is, he entered an Ohel in which there was a Goses (one about to die), and the Goses died when he was there. Tum'ah and entering come together.

Shevuos 17a - Question (Rav Ashi): If a Nazir in a cemetery delays leaving less than the time to bow, is he lashed?

Nidah 57a - Question: Why is a Kusi believed to say that there is no Tum'as Ohel in a certain place? He is not concerned for "v'Lifnei Iver Lo Siten Michshol"!

Answer (R. Avahu): A (Kusi) Kohen is believed about the place he is standing.

Question: Perhaps he is already Tamei!

Answer: He is eating Terumah.

Berachos 19b (Rav bar Sheva and Rav Kahana): Chachamim said that Kavod ha'Beriyos (human dignity) overrides all mid'Rabanan laws.

Rishonim

Rambam (Hilchos Nezirus 5:16): If a Nazir was Mitamei to a Mes many times, even though he liable to Shamayim each time, Beis Din lashes him only once. If they warned him each time, he is lashed for each (warning).

Rambam (17): If he was still touching the first Mes when he touched the second, he is liable only once.

Rebuttal (Ra'avad): This is like Rav Yosef. The Halachah follows Rabah against Rav Yosef. He is exempt even if he separated from the first Mes before touching the second. Nowadays, Kohanim are Temei'im, so they have no Chiyuv for Tum'ah. One who says that they do must bring a proof.

Kesef Mishneh: The Rambam holds that we always follow Rabah against Rav Yosef only in Bava Basra. Since Rav Yosef swore, surely he was sure about the Halachah.

Lechem Mishneh: The Rambam agrees that the Halachah follows Rabah, but he explains the Sugya like Tosfos. Abaye distinguished between connected and separate Tum'os to resolve the Mishnah and Beraisa. He holds like Rabah. The Halachah follows him, for Rav Yosef was refuted. The Ra'avad holds like Rashi, that Rav Yosef himself answered. Surely, one is liable only once for Tum'ah when they are connected. Rav Huna obligates twice in any case, and Rabah never obligates twice. They are wrong. Rabah must explain that the Mishnah obligates twice for Bi'ah and Tum'ah at the same time, when he was warned for both. Each Perush has advantages and difficulties.

Chazon Ish (Aveilus 210:13): Rashi (42b DH Aval exempts only if he is still touching the first Mes when he touches the second. This) is unlike the Ra'avad. The Ra'avad exempts only nowadays, when we do not have ashes of the red heifer, so touching Tum'ah does not delay his Taharah in any case.

Kesef Mishneh (Hilchos Evel 3:7): The Ra'avad holds that Rabah obligates only once even for separate Tum'os. This is wrong. To resolve the Mishnah and Beraisa, all must obligate twice, but only if they are separated!

Lechem Mishneh (Hilchos Evel 3:7): The Kesef Mishneh retracted from his Perush in Hilchos Nezirus.

Migdal Oz: The proper text says that Rava cited Rav Huna. Rav Yosef swore that Rav Huna said differently, and he answered Abaye's question. The Sugya concludes like Rav Yosef; we follow him against his Talmid Abaye.

Note: If Rava cited Rav Huna, why is his opinion brought before that of Rav Yosef, who preceded Rava? Perhaps this is to explain why Rav Yosef needed to swear, to firmly rebut Rava. The Rashbam (Bava Basra 114b DH mi'Shum) says that 'Omar' connotes citing one's Rebbi. Rava was not Rav Huna's Talmid, for Rava was born when Rav Yehudah died (Kidushin 72b), i.e. after Rav Huna died (Igros Rav Shrirah Gaon, Rabanan Savorai 94).

Rambam (Hilchos Evel 3:7): If a Tamei Kohen entered Ohel ha'Mes, he is lashed even for entering.

Rebuttal (Ra'avad): Rabah does not obligate even for separate Tum'os, all the more so for connected Tum'os! He obligates for Tum'ah and Bi'ah only if they came at the same time.

Rosh (Hilchos Tum'ah 6): A Kohen is liable for a second Tum'ah only if it is after he separated from the first. The Sugya in Nazir is unlike R. Akiva (Evel Rabsi 4:15-16), who exempts one who was Mitamei the same day that he separated from Tum'ah. R. Tarfon obligates. All obligate for Tum'ah on a later day, for he delays his earliest possible Taharah. The Halachah follows R. Tarfon; the Stam Mishnah (Nazir 42b) and Shevuos 17a are like him. They argue only about Chiyuv, but all forbid (Evel 4:13-14). Therefore, a Kohen buries a relative at the edge of the cemetery, and not in the middle (Rambam Evel 2:15). He may be Mitamei to others while holding his Mes, but not after burial. R. Tam permits him to bury in the middle, for the Halachah follows R. Akiva. Even if R. Akiva forbids, this is only mid'Rabanan, and Kavod ha'Briyos overrides it. He learns from Nidah 57a. We suspect that a Kusi Kohen is already Tamei, and therefore he may stand over Tum'ah. How can R. Tam bring a proof from Kusim? Perhaps they follow R. Akiva, or expound "to become Tamei" to exclude someone already Tamei!

Poskim

Shulchan Aruch (YD 373:7): A Kohen may not be Mitamei to a Mes, even while being Mitamei for a relative. Therefore, he must bury his relative at the edge of the cemetery, lest he be Mitamei through others while burying his Mes.

Rema: This is only after separating from the Tum'ah. While engaged in his Mes, he may be Mitamei to others.

Shach (12): The Shulchan Aruch and the Bach are stringent even on the way to bury his Mes, for it is possible to bury at the edge.

Suggestion: A Sefek-Sefeka permits a Kohen to tower over dead Nochrim. Perhaps the Halachah follows the Ra'avad. And even if not, perhaps the Halachah follows R. Shimon.

Retraction (Dagul me'Revavah): Perhaps the Ra'avad only exempts a Tamei Kohen from lashes, but even he forbids exposing himself to Tum'ah.

Chasam Sofer (YD 338 DH u'Mah she'Chosav and 239 DH Lich'orah): In Tamim De'im (236, on Makos 21a), the Ra'avad holds like the Rambam. There were two Ra'avad's. R. Tam, Rashi and the Re'em also exempt a Kohen who was Mitamei again on the same day. In a case of Safek danger we may rely on them to touch a dead Nochri, for then we may join the Re'em's opinion that Nochrim do not have even Tum'as Maga.