Why does it say "Nachamu"?
Rashi: He returns to future Nevu'os. From here until the end of the Sefer are consolations. You, My Nevi'im, should console (Targum Yonasan - have Nevu'os of consolation of) My nation.
Radak: All the consolations are for the days of Mashi'ach.
Why is "Nachamu" repeated?
Vayikra Rabah 10:2: Yeshayah accepted to be Hashem's Shali'ach to Yisrael, even if they will hit him. Hashem said, since you wanted to vindicate My children, and not obligate them - the other Nevi'im had simple Nevu'os, and you will prophesize double consolations - "Uri Uri", "His'oreri His'oreri", "Sos Asis", "Anochi Anochi Hu Menachemchem", "Nachamu Nachamu" (51:9, 12, 61:10, 51:12, 40:1).
Eichah Rabah (1:57): Yerushalayim sinned doubly - "Chet Chat'ah Yerushalayim" (Eichah 1:8), it was stricken doubly - "Ki Lakchah mi'Yad Hashem Kiflayim b'Chol Chatoseha" (below, 2), and it is consoled doubly - Nachamu Nachamu.
Radak: The double expression strengthens the matter.
Malbim: Redemption can come before its time, if they merit, or if they receive their punishment, or in any case, even if they do not merit, He will redeemed them in the fixed time. Therefore, it is doubled.
