Why must he say "Odeni ha'Yom Chazak", in addition to "k'Kochi Az uch'Chochi Atah"?
Malbim: The former teaches that my heart is strong; it does not fear anything [physical].
What do we learn from "k'Kochi Az uch'Chochi Atah"?
Radak: This equates them, like "cha'Am ka'Kohen ka'Eved ka'Donav" (Yeshayah 24:2).
Malbim: The Chidush (which we learn that it is like the other) comes first, e.g. "Chamocha k'Pharaoh" (Bereishis 44:18)! Kalev's strength was known now in the war; he estimated that it was the same 45 years ago. 1