Rashi on Isaiah
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.
Malbim on Isaiah
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.
Metzudat David on Isaiah
Comfort, Comfort. Your G-d says to the prophets "Comfort my people", and the doubling of the word alludes to the stress of the command.