Bava Batra 123 - The Double Portion of the Firstborn
Is the double portion of the firstborn simply double that of every other brother, or does he take 2/3 of the whole estate, with the other brothers dividing the remaining 1/3?
The answer comes from the phrase "... and it shall be on the day that he bequeaths to his sons ..." This phrase is superfluous, since the law it states - that the firstborn gets a double portion - is expressed in different words in the very next verse. Therefore, the superfluous phrase teaches that the other brothers - if there are more than one - get at least as much as the firstborn, that is, at least 1/2, which is more than 1/3. Now that the 2/3 possibility is out, the only remaining logical alternative is that the firstborn gets double the portion of each brother.
Art: Thomas Hudson - The Courtenay Brothers