To men: If somebody is in the middle of Birkat Hamazon (Grace after Meals) and remembers that he already said Birkat Hamazon, what is he supposed to do?
There’s a similar Gemara discussion, but it talks about the Amida (standing prayer). The Gemara discusses Tefilat Nedavah (voluntary prayer) in Masechet Berachot (Tractate Berachot): a person is allowed to add an extra Amida if he wants to do a Nedavah (voluntary offering). So the Gemara asks, what happens if somebody is in the middle of the Amida and remembers that he already said the Amida? The Gemara says, “Posek miyad”: he stops immediately, right there.
But you could have a Kavanah (intention) for Tefilat Nedavah. No. The Gemara says it’s only when your Kavanah for Tefilat Nedavah was in place before you started praying. But if you’re in the middle of praying and thinking that you’re doing Chovah (obligation), and in the middle you want to switch from Chovah to Nedavah, it doesn’t work. So, posek miyad; that’s the Halakha (Jewish law).
The Gemara says there’s an exception to the rule: Tefilat Arvit (evening prayer). Arvit (evening prayer), since Tefilat Arvit is Reshut. There’s a Machloket Rishonim (dispute among early authorities): what does it mean that Tefilat Arvit is Reshut? Everybody thinks Tefilat Arvit being Reshut means that it’s optional. Tosfot (Tosafot commentary) says no. Reshut means that if you have another mitzvah (commandment), the other mitzvah is first. That’s what it means. If you have no other mitzvah, of course, 100% Arvit is Chovah.
In comparison with another mitzvah, you don’t say anymore, “Oh, I’m osek b'mitzvah, patur min hamitzvah (one engaged in a mitzvah is exempt from another). I’m osek b'mitzvah (engaged in a mitzvah) at Arvit.” No, no, no. That’s only in a regular mitzvah. In this mitzvah, you always prioritize another mitzvah over Arvit. That’s what Tosfot says.
Anyway, coming back to this: since it’s based on Nedavah, it’s similar to Nedavah. I’m forcing myself to make a Nedavah every evening. Therefore, the Gemara says that Tefilat Arvit is Reshut. If the mistake happened in the Amida of Arvit, you could continue the Amida. If you prayed Arvit already and remember in the middle of Arvit that you already prayed, it’s fine, because you started with the idea of Arvit, which is Reshut, which means Nedavah. Therefore, you could continue in Nedavah mode. That’s what the Gemara says.
But back to the regular Amida. If you remember in the middle, posek miyad. Birkat Hamazon is the same thing, because there’s no such thing as Nedavah in Birkat Hamazon. It doesn’t exist. It’s only Chovah. If a person in the middle of Birkat Hamazon remembers he already said Birkat Hamazon, there are no exceptions. He has to stop immediately. Of course, we know he says Baruch Shem Kevod Malchuto le'olam va'ed (Blessed is the name of His glorious kingdom forever and ever), but he has to stop immediately.