Adam Clarke's Commentary on the Bible

Verse 2 (Isaiah 40:2)

Double for all her sins "Blessings double to the punishment" - It does not seem reconcilable to our notions of the Divine justice, which always punishes less than our iniquities deserve, to suppose that God had punished the sins of the Jews in double proportion; and it is more agreeable to the tenor of this consolatory message to understand it as a promise of ample recompense for the effects of past displeasure, on the reconciliation of God to his returning people. To express this sense of the passage, which the words of the original will very well bear, it was necessary to add a word or two in the version to supply the elliptical expression of the Hebrew. Compare Isaiah 61:7 ; Job 42:10 ; Zechariah 9:12 . חטאה chattaah signifies punishment for sin, Lamentations 3:39 ; Zechariah 14:19 . But Kimchi says, "Double here means the two captivities and emigrations suffered by the Israelites. The first, the Babylonish captivity; the second, that which they now endure." This is not a bad conjecture.

- Adam Clarke's Commentary on the Bible