Why did Moshe repeat "Va'esnapal... Arba'im Yom
Rashi: In order to add the basic text of his Tefilah there, which the Pasuk goes on to quote. 1
Rashbam: He repeated it here as part of the rebuke. He pointed out to the people that he succeeded in preventing their immediate annihilation due to Chilul Hashem, in case people say that Hashem was incapable of conquering the Cana'anim. But once they enter Eretz Cana'an and defeat the Cana'anim, this will no longer apply, so if they sin then, nothing will prevent Hashem from destroying them. 2
Refer to 9:22:2:1-3.
B'rachos, 34a: To teach us that when necessary, no Tefilah is considered too long. 3
Oznayim la'Torah: To add that, in spite of the fact that the people allowed Moshe to burn the calf and punish the main sinners, a sign that they had done Teshuvah, it did not suffice 4 to pardon Yisrael completely until he Davened on their behalf for another forty days, to annul the decree of annihilation.
See Rashbam, who queries this explanation, and who asks why the Torah did not simply insert the text in Pasuk 18? See also Oznayim la'Torah, who asks why such a short Tefilah should require forty days and forty nights? See Oznayim la'Torah citing the Ha'amek Davar, who answers it, and note on answer #4.
