Before the "Torah" included five books, it was only the book of Deuteronomy!
To the extent that Deuteronomy exhibits a certain overlap with other Pentateuchal passages, Deuteronomy should NOT be seen as mere repetition of the first four books of the Bible, or even as an attempt by Deuteronomy to clarify these books.
Rather, Deuteronomy’s purpose when referencing other Pentateuchal legal and narrative passages should be understood as an attempt to modify, or even radically (if subtly) transform, its earlier or contemporary sources.
It does so in a self-contained torah: (an early form of) the book of Deuteronomy.
by Dr. David Glatt-Gilad - Ben-Gurion University of the Negev