Reish Lakish citing Abba bar Dalayah: (Why does a firstborn receive a double portion of his grandfather's estate if his father was a firstborn?) The pasuk uses the word "mishpat'' (justice) about the double portion and uses the word "mishpat'' about the regular portion...just as for the regular portion, you view the son as if he is alive to take the regular portion of his father; so for the double portion, you view the son as if he is alive to take the double portion.