כִּי֩ אֶת־הַבְּכֹ֨ר בֶּן־הַשְּׂנוּאָ֜ה יַכִּ֗יר לָ֤תֶת לוֹ֙ פִּ֣י שְׁנַ֔יִם בְּכֹ֥ל אֲשֶׁר־יִמָּצֵ֖א ל֑וֹ כִּי־הוּא֙ רֵאשִׁ֣ית אֹנ֔וֹ ל֖וֹ מִשְׁפַּ֥ט הַבְּכֹרָֽה׃ (ס)
반드시 그 미움을 받는 자의 아들을 장자로 인정하여 자기의 소유에서 그에게는 두 몫을 줄 것이니 그는 자기의 기력의 시작이라 장자의 권리가 그에게 있음이니라
Rashi on Deuteronomy
פי שנים A DOUBLE PORTION — i.e. a portion equal to those of two brothers together (Sifrei Devarim 217:2-4).
Rashbam on Deuteronomy
כי את הבכור, the word כי means אלא, but. כי הוא ראשית אונו, for he is entitled to inherit the first of his father’s money. We find that the word און appears in this sense in Hoseah 12,9 מצאתי און לי, [clearly it cannot mean “I have found my virility there”, as it does in Genesis 49,3. Ed.] The word און also occurs in the sense of money in Job 20,10 וידיו תשיבנה אונו, “his own hands must give back his wealth.”
Siftei Chakhamim
Equivalent to two brothers. Rashi is answering the question: “Doublefold of all that he possesses” implies doublefold of everything he possesses, i.e. that he should have two portions. [For example,] if he left three manah and there were five brothers, the firstborn should take two manah and the four [remaining] brothers should take one [manah]! Rashi answers that “of all he possesses” does not refer to “doublefold” but means as follows: “Doublefold [of one brother’s share],’ i.e., the equivalent of what two brothers [take].” And [the term] “of all that he possesses” is a separate phrase.