“Devote them to destruction, yes, destruction.” God commanded that the nations inhabiting Canaan be completely destroyed, and He emphasized that fact by using the figure of speech polyptoton, where a verb is repeated twice but with different inflections, in this case, the verb “devote to destruction.” The Canaanite nations had been genetically corrupted by Nephilim, and could not be godly or redeemed. They had to be destroyed.
[For more on polyptoton, see commentary on Genesis 2:16.]
[For information on the Nephilim, see commentaries on Genesis 6:2 and 6:4.]