Or HaChaim on Exodus 17:14/אור החיים על שמות י״ז:י״ד

1 ושים באזני יהושע, "and read it aloud to Joshua, etc." We need to understand why G'd instructed Moses to address this message only to Joshua. Perhaps the reason was the fact that despite Joshua's valiant efforts to overcome Amalek he had succeeded only in weakening him. Joshua became unduly concerned that G'd had not wiped out Amalek as He had wiped out the Egyptian army. G'd therefore wanted to reassure him. By telling Moses to be sure to make Joshua hear this message, G'd wanted to tell him that He, G'd, had heard Joshua's unspoken concern. He told Joshua not to worry, that in due course He would wipe out Amalek completely from beneath the sky.

2The repetition of the words מחה אמחה, may be an allusion to two occasions when a major battle would be fought against Amalek, one by King Saul, the other by Mordechai. The third and last such battle will occur prior to the coming of the Messiah.