Why is "Hamonim" repeated?
Radak: It is to strengthen the matter.
Why is it called "Emek ha'Charutz"?
Rashi: There it was finalized that the judgment is Emes.
Radak #1: There, the nations will be eradicated and cut.
Radak #2: There, their judgment will be decided, just like it is called Emek Yehoshafat.
Malbim: Multitudes will gather there because it will be "Emek ha'Charutz." It is called "Emek Yehoshafat" at the time of Mishpat - then He investigates their law and judges to know if they are innocent or liable. Now it is called "Emek ha'Charutz", for their judgment was already decided that they are liable to be eradicated. Therefore "Karov Yom Hashem", and He will quickly eradicate them, since the verdict is final.