Some context for Deuteronomy.
Deuteronomy was written by Moses; "to all Israel across the Jordan in the wilderness, in the Arabah opposite Supha, between Paran and Tophel and Laban and Hazeroth and Dizahab."
The setting is this: Moses is giving final addresses in the last days of his life. The Israelites where getting prepared to go into the promised land. The people were facing war, temptations, and a new, settled way of life-all under the unproved leadership of Joshua. This generation of Israelites had not personally experianced the red sea or the giving of the law at Sinai. Moses was reminding them of God's power and God's laws.
application: If God found it nessaccary to remind his people twice and is written down twice in the bible, I had pay double attention!