Why did he say twice "Aneni"?
Rashi citing Targum Yonasan: Answer me with fire, and answer me with rain. Radak - even though he would pray for rain later in the day, he included it now in his prayer.
Rashi citing Brachos 9b: Answer me with fire, and answer me, that people will not say that this was witchcraft 1 . Malbim - i.e. that the fire descended through the power of a lower Sar, who is the Sar of Yisrael.
Radak: Whenever there is no reason for the repetition, it is to strengthen the matter. The same applies to "Kel Elokim Hashem Kel Elokim Hashem" (Yehoshua 22:22). All who pray, the norm is to repeat matters a second or third time, e.g. "Hashem Shema'ah Hashem Selachah Hashem Hakashivah" (Daniel 9:19).
Radak citing Yerushalmi Ta'anis 2:8: Answer me in my merit, answer me in the merit of my Talmid. 2
Malbim (Shemos 8:7): How does Brachos 6b learn from our verse that one must be careful about Tefilas Minchah? Perhaps Eliyahu prayed now, for until now Nevi'ei ha'Ba'al called to Ba'al! The Zohar says that witchcraft works only from midday until midnight. Our verse teaches that Eliyahu feared lest people say that he acted through witchcraft. He should have offered before midday, to avoid the concern! Rather, he waited until after midday due to the power of Tefilas Minchah. (They will know via the water that it is not witchcraft (refer to 18:34:2:2)! Perhaps all knew that witchcraft does not work in the morning, but they did not know that water nullifies it. Or, perhaps they will say that it was a clear liquid other than water (e.g. alcohol). However, above (verse 26), Malbim wrote that Ba'al was Avodah of the sun; it is strongest from morning until afternoon. If so, this is why he let them call to the Ba'al until the afternoon! - PF)
Why does it say "v'Yed'u... Ki Atah Hu Hashem Elokim"?
Malbim: Even though they will understand that You are Elokim amidst them, and I am Your servant, still they will not know that Elokei Yisrael is the One being who must exist, who caused everything to come into existence and keeps it existing. They will err to think that [Elokei Yisrael] receives His power from the Supreme source.
What is the meaning of "v'Atah Hasibosa Es Libam"?
Rashi: You gave to them place (opportunity) to veer from You. You can return their hearts to You!
Rashi citing Bamidbar Rabah 18:12: If you will not answer me, I will deny and say that You turned their hearts [away from You]. Moshe said similarly "Im k'Mos Kol Adam Yemusun Eleh" (Bamidbar 17:29), also I deny and say that Hashem did not send me to say the Torah and Mitzvos. Michah said similarly "Im Shov Tashuv b'Shalom Lo Diber Hashem Bi" (below, 22:28)
Radak citing Rav Sadya Gaon: If You will answer me, their hearts, which were backwards (away from You) will return to You.
Radak #1: [If You will answer me,] their hearts will turn away from believing in the Ba'al; they will know that it is falsity.
Radak #2: If You will answer me, they will know that You alone are Elokim. You did not show them an open wonder until now so that they will recognize that You are Elokim; it seems that You turned their hearts back [from You].
Radak, citing Brachos 31b: You should have prepared their hearts for good. 1
Radak citing the Rambam (Hilchos Teshuvah 6:3): You prevented them from doing Teshuvah, in order that they will receive their punishment. This is like "va'Ychazek Hashem Es Lev Pharaoh" (Shemos 9:12) and "Hiksheh Hashem Elokecha Es Rucho v'Imetz Es Levavo" (Devarim 2:30) and similar verses.
Radak, citing Targum Yonasan: This nation will know, when You do a miracle [for them, that You are Hashem Elokim, and in Your love for them You will seek for them with Your word to return them to Your fear, and they made their hearts divided].
Malbim: The reason why Yisrael err and do not understand Your Elokus is because You created man in a way that he does not understand from before to later via intellect, only from later to earlier, from the result to the cause. Therefore, he is prone to err; the mistake is in matters of investigation,
Radak: One should not rely on this Perush. People have free choice, like it says "Nasati Lefanecha ha'Yom Es ha'Chayim v'Es ha'Tov Es ha'Maves v'Es ha'Ra" (Devarim 30:15). A person chooses what he wants! Also Chachamim said 'everything is in the hands of Shamayim except for fear of Shamayim' (Brachos 33b).
