Getting a Second Opinion

Question: I asked my rav about fasting on shiva asar betammuz and he told me that I should not fast. I then went and asked a second rav and he told me that I should. What should I have done?

Answer: The Gemara (Avoda Zara 7a) teaches that after asking one Rabbi whether something is permissible, and being told that it is prohibited, one may not approach a second Rabbi to ask him if it is permissible. Elsewhere, the Gemara (Nidda 20b) teaches that one Rabbi may not give a different ruling to one who had previously asked another Rabbi.

There are different reasons given for this prohibition. Rashi (Nidda 20b) explains that the second Rabbi must act respectfully towards the first Rabbi. Tosafos (Nidda 20b) adds the prohibition is specifically on the Rabbi. However, other poskim understand that the prohibition is primarily on the one asking. The Rosh (Avoda Zara 1:3) and Raavad (quoted by the Ran, Avoda Zara 7a) explains that when the first Rabbi paskens that an item is assur, it becomes assur, and the second Rabbi cannot undo this (unless the first Rabbi was mistaken). The Ran adds that when one receives a second ruling, it may appear as if there are two separate laws, and the Torah is inconsistent, God forbid.

Tosafos (Avoda Zara 7a) and the Rema (YD 242:31) write that the prohibition only applies if one did not tell the second Rabbi that they initially consulted with another Rabbi. If one mentioned that one had already asked a Rabbi, then it is permissible to ask a second Rabbi. If the second Rabbi has a different opinion, though the first Rabbi was not mistaken, the second Rabbi should discuss it with the first Rabbi, rather than undermine him (See Betzel Hachachma 6:29).

In conclusion, one may only ask a Rabbi for a second opinion, if they tell the second Rabbi that they already asked the question to another Rabbi.