Gemara

(Mishnah): A Neder takes effect on another Neder, but a Shevuah does not take effect on a Shevuah;

If one said "I will be a Nazir if I eat, I will be a Nazir if I eat", and he ate, he must conduct one Nezirus for each time he said so;

If he said "Shevuah, I will not eat this. Shevuah, I will not eat this" and he ate it, he is liable only once.

(Gemara - Rav Huna): This is only if the latter Nezirus finishes later, e.g. he said "I will be a Nazir today (if I eat), I will be a Nazir tomorrow..." If he said "I will be a Nazir today, I will be a Nazir today," the latter Nezirus does not take effect.

(Shmuel): Even if he said "I will be a Nazir today, I will be a Nazir today", both take effect.

Question: What is the case in the Mishnah?

Answer #1: The Shevuos were "I won't eat figs, I won't eat figs." The corresponding Nedarim were "I will be a Nazir today, I will be a Nazir today."

The Mishnah says that both take effect. This refutes Rav Huna!

Answer #2: Really, he said "I will be a Nazir today, I will be a Nazir tomorrow. The corresponding Shevuos were "I won't eat figs, I won't eat figs and or grapes." The second oath does not take effect.

Question: The second oath takes effect!

(Rabah): If one swore "I won't eat figs. I won't eat figs and grapes", and he ate figs, and was Makdish a Korban for this, and then he ate grapes, the latter transgression is like Chetzi Shi'ur (a partial Isur), for which one does not bring a Korban.

Had he not been Makdish a Korban, he would be liable also for the second oath.) Since the second oath takes effect on grapes, it takes effect also on figs!

Answer: Rav Huna disagrees with Rabah.

(Mishnah): If he said "Shevuah, I will not eat this. Shevuah, I will not eat this" and he ate it, he is liable only once.

Version #1 (Rava): If he permitted the first oath, the second takes effect.

He derived this from the Mishnah. "He is liable only once" connotes that there is no room for the second oath to take effect, but when he permits the first (and there is room), it takes effect.

Version #2 - Inference: He is not liable for the second, but it is a Shevuah.

Question: In what sense is it a Shevuah?

Answer: This is like Rava taught, that if he permits the first, the second takes effect.

Shevu'os 24b (Rava): According to the opinion that Isur Chal Al Isur (another Isur can takes effect on something forbidden) if the latter Isur is Kolel (forbids more things), if one swore 'I will not eat figs', and later swore 'I will not eat figs and grapes', since the latter oath takes effect regarding grapes, it takes effect also regarding figs.

Objection: This is obvious!

Answer: One might have thought that an Isur Kolel of the Torah is Chal Al Isur, but an Isur Kolel (such as an oath) that a person made by himself is not. Rava teaches that this is not so.

Rishonim

The Rif brings both versions of Rava.

Ritva (DH Gemara Omar): It seems that even when he permits the first Shevu'ah, he is lashed for the second, for when he said it was vain. Even though warning for lashes cannot be mi'Safek, this is not considered Safek warning, for the second oath is vain until an action (Heter) dependent on others is done.

Rambam (Hilchos Shevu'os 4:9): If one said "Shevuah, I will not eat this loaf. Shevuah, I will not eat it" and he ate it, he is liable only once.

Rambam (11): If one swore not to eat figs, and then not to eat figs and grapes, he is liable twice for figs. Since the second oath takes effect on grapes that were permitted, it takes effect also on figs that were forbidden.

Radvaz: The latter takes effect even regarding lashes and Korban.

Rosh (Nedarim 2:8): Rava taught that if he permits the first, the second takes effect.

Rosh (Shevu'os 3:8): Rava teaches that Kolel applies even when a person made both Isurim.

Poskim

Shulchan Aruch (YD 238:6): If one swore not to eat figs, and then not to eat figs and grapes, both Shevu'os take effect on figs.

Rema: If one swore to be Mekadesh a woman before Pesach, if she will want, and then not to be Mekadesh her, the latter takes effect through Kolel, because he is not obligated to be Mekadesh her after Pesach. If the first Shevu'ah was for her benefit or through Shiduchin, he cannot uproot her rights. Therefore, he must permit the latter oath in order to fulfill the first (Beis Yosef citing Teshuvas Ran 74).

Taz (4): The Ran discussed when he agreed to be Mekadesh her when Ploni will request, before Pesach. It seems that the first Shevu'ah benefits her even without Shiduchin, for he must be Mekadesh her if she desires! Even if he does not seem like a proper match for her, perhaps she desires him.

Rosh (Teshuvah 8:2, cited in Beis Yosef DH Kosav ha'Rosh): Reuven swore not to sell his property to pay his debt to Shimon, and later swore to pay him by a certain day. The day came, and he did not have the money. He fulfills his first oath. The latter oath is Shav, and he is lashed mid'Rabanan for it. If the lashes cause him to regret his first oath, he permits it and fulfills his second oath. This is not a Pesach of Nolad (an unexpected development), for it is known that one is lashed mid'Rabanan for swearing. This is only if the first oath preceded the loan. If not, the first oath did not take effect, for it is a Mitzvah to pay debts. His oath does not uproot the oath of Sinai. If he does not want to pay, we beat him until he dies.

Shulchan Aruch (21): If one swore twice not to eat a loaf, the latter oath does not take effect. If he permitted the first, the latter takes effect.

Taz (229:10): The Shulchan Aruch holds like the Rosh and Tur, that the latter Shevu'ah does not take effect at all until the first is permitted, therefore he cannot permit the latter before permitting the first. The Rambam (Shevu'os 6:17) disagrees. Maharlbach explains that this is because the Shevu'ah is in the world, just there is no room for it to take effect.

Pores Yosef (18a DH Lishna): According to Version #1, he cannot permit the second Shevuah before the first, for it merely a potential Shevuah.

Shach (26): Mahari Mintz (40) says that Hataras Chacham works retroactively, therefore when the first oath is uprooted, the second takes effect. Hafaras ha'Ba'al is not retroactive, so after Hafarah of the first, the latter does not take effect.

Shulchan Aruch (239:13): A Shevu'ah does not take effect on a Shevu'ah. If one swore twice not to eat a loaf, the latter does not take effect. However, if he permitted the first oath, the second takes effect.