Or HaChaim on Exodus 15:16/אור החיים על שמות ט״ו:ט״ז

1 הפול עליהם אימתה ופחד, "Terror and dread will fall upon them, etc." Both this verse and the one following it refer to two separate conquests of the Holy Land, one during the period of Moses/Joshua and the other at the time when the Messiah will arrive. This is the reason the author of the song appears to repeat what he says.