נַחֲמ֥וּ נַחֲמ֖וּ עַמִּ֑י יֹאמַ֖ר אֱלֹהֵיכֶֽם׃
טרײסט, טרײסט מײַן פֿאָלק, זאָגט אײַער גאָט.
Rashi
Console, console My people He returns to his future prophecies; since from here to the end of the Book are words of consolations, this section separated them from the prophecies of retribution. Console, you, My prophets, console My people.
Ibn Ezra
This chapter1In this remark I. E. seems to consider the whole book of Isaiah as one, and to explain only the connection between the thirty-ninth chapter and the fortieth, without anticipating the question concerning the authorship of the second part, or concerning the period to which c. xl. refers. The prediction of the Babylonian exile (c. xxxix.) is, according to the opinion of I. E. properly followed by prophecies of comfort and happiness, even if those announce the release from some other exile. A similar remark is made by I. E. on the position of the thirteenth chapter. has been placed here for the following reason: in the preceding chapter it is predicted that all the treasures of the King, and even his sons, will be carried away to Babylon; this sad prediction is properly followed by the words of comfort.
Malbim
Comfort, comfort. You, prophets, comfort my people. And since the redemption will come either before its time because of merit, or if they receive complete punishment for their sins, or even if they aren't meritorious, He will redeem them at the set time, therefor it said "comfort" twice.