Miriam's closing echo of this exact same line -- word for word -- bracketing the whole Song of the Sea.
Then Moses and the children of Israel sang this song to Yahweh, and said, “I will sing to Yahweh, for he has triumphed gloriously. The horse and his rider he has thrown into the sea.
Miriam answered them, “Sing to Yahweh, for he has triumphed gloriously. The horse and his rider he has thrown into the sea.”
Many scholars date this poem's core language among the oldest surviving Hebrew in the Bible, based on its archaic grammar and vocabulary - meaning the story of the Exodus may have been sung, in something close to this form, long before Exodus was written down as a narrative at all. The prose account (ch.14) and this poetic retelling (ch.15) aren't redundant; the poem is doing something the story doesn't - turning the event into worship, meant to be sung rather than simply reported.