What is the meaning of "Ro'u"?
Rashi: It is an expression of Re'ehu, Re'acha (your colleague). This is like Targum Yonasan - join together, the multitudes of Sancheriv.
Radak: It is an expression of breakage, like "Tero'em b'Shevet Barzel" (Tehilim 2:9). This addresses the nations that gathered with Melech Ashur; break [others], and afterwards you will be broken.
Malbim: He addresses the conspirators (refer to 8:5:1:2) - join with the nations, and make a Bris to rebel against your masters! (However, you will be broken.)
What is "va'Chotu"?
Rashi: You will be broken.
Why does it say "His'azeru va'Chotu"?
Rashi: Gird and invigorate yourselves with all kinds of Gevurah, and [even so] in the end you will be broken.
Malbim: Invigorate yourselves to rebel, but you will be broken.
Why is "His'azeru va'Chotu" repeated?
Radak: The matter is doubled, to strengthen it.
Malbim: Invigorate yourselves again to rebel, but it will not help; you will be broken.
