1)
Why did they say "Hashem Hu ha'Elokim"?
1.
Rashi: [Only he is ha'Elokim;] the Ba'al is not Elokim.
2.
Refer to 18:39:2:2.
2)
Why did they repeat "Hashem Hu ha'Elokim"?
1.
Radak: This was to strengthen the Emunah in their hearts.
2.
Malbim: The first was positive - Hashem is Elokim; they saw this from the fire that descended. The second was to exclude - nothing else is Elokim. They saw that the Ba'alim are futile.
