Or HaChaim on Genesis 27:37/אור החיים על בראשית כ״ז:ל״ז

1 ויען יצחק, Isaac replied. Actually Isaac still had the chance to bless Esau. Having realised in the meantime that Esau deserved a curse rather than a blessing, that he was in fact an enemy of his brother Jacob, Isaac was afraid to bless Esau because he would thereby neutralise the blessing to Jacob that those who curse him would themselves be cursed. The Torah therefore mentions the expression ויען יצחק twice to tell us that he did not tell Esau the true reason why he could not bless him outright.