Why does it say "va'Yomer" twice?
Rashi (from Vayikra Rabah 26:8): Always, we must expound this. The first Amirah (message) was, I will give this gigantic assembly into your hands. The second 1 was, if Ben Hadad falls to you, do not have mercy on him. Therefore, when Achav had mercy on him, he was rebuked and punished (verse 42).
Radak: He repeated this to him, to strengthen in his heart that the salvation will be from Hashem. Similarly "va'Yomer ha'Melech va'Yomer l'Esther" (Esther 7:5), "va'Yomer Yitzchak El Avraham Aviv va'Yomer Avi 2 " (Bereishis 22:7) [are to strengthen the matter].
Malbim: The first Amirah was about matters of the war, not in the name of Hashem. I.e. he reminded him that last year, he told him to prepare strategy and Gevurah [for a second war at the end of the year], and he did not prepare properly. The second, in the name of Hashem, was that based on Achav's merit, He should not help him now. Even so, He will give Ben Hadad into Achav's hand again because Ben Hadad denied Hashem's power, and said that He is G-d [only] of the mountain.
Also Radak says so. Malbim says that this was the first Amirah. All the Midrashim say so; the second was, 'know how many traps I set for him until he came to your hand.'
Why did his question need to be strengthened? Perhaps this is like the Midrash that Yitzchak appealed to Avraham's mercy not to sacrifice him (refer to Bereishis 22:7:151:1 and the note there)!
Why did he say "Ya'an Asher Amru"?
Radak: [The victory] will not be due to your virtue and straight heart. You did not abandon the Ba'alim after seeing the great salvation that Hashem did for you amidst your evil! Rather, Hashem will do a miracle for you to sanctify His name, for they said that He is G-d of the mountain but not G-d of the low places.
Why will they know now that Hashem is Elokim? Hashem already saved them!
Radak: Perhaps they thought that the first time was chance. Now you can know that the salvation was from Hashem.
