(Mishnah): If one knew of the Tum'ah, and later forgot (and entered the Mikdash or ate Kodshim), and later remembered that he was Tamei, he brings an Oleh v'Yored (i.e. a rich person brings an animal, a poor person brings birds, and a very poor person brings a flour offering);

If he knew of the Tum'ah, and forgot (and entered the Mikdash or ate Kodshim), and did not find out at the end [before Yom Kipur], the goat (whose blood is) offered inside (the Kodesh ha'Kodoshim on Yom Kipur) and Yom Kipur itself protect him from punishment until he finds out, and then he will bring an Oleh v'Yored.

(Gemara) Question: What is the source that [he brings a Korban only if] there was Yedi'ah at the beginning and end, and Helam in the middle?

Answer #1: It says "v'Nelam...; v'Nelam" twice [and v'Yada in between]. This implies that there was Yedi'ah [before and after forgetting, i.e.] at the beginning and end, and Helam in the middle.

Question: This is like R. Akiva. How can we answer for R. Yishmael? (He uses the repetition of v'Nelam to obligate for Helam Mikdash, even if he remembered the Tum'ah.)

Answer: R. Yishmael expounds like Rebbi "v'Nelam Mimenu" implies that he knew. It says also "v'Hu Yada" - these are two Yedi'os.

Question: R. Yishmael expounds like Rebbi. Must Rebbi hold like R. Yishmael?

Answer: [Rebbi] can hold even like R. Akiva. The repetition of "v'Nelam" is needed to teach Yedi'ah and Helam for Tum'as Mikdash, and Yedi'ah and Helam for Tum'as ha'Kodesh.

Answer #2: Some learn from "v'Hu Yada v'Ashem." (Why is this needed?) It already says "v'Hu Tamei v'Ashem"!

Rather, Im Eino Inyan (since it need not teach) Yedi'ah at the beginning, we use it to teach about Yedi'ah at the end.