John Calvin Commentary Genesis 28:1

"And Isaac called Jacob, and blessed him, and charged him, and said unto him, Thou shalt not take a wife of the daughters of Canaan." — Genesis 28:1 (ASV)

And Isaac called Jacob, and blessed him. It may be asked whether the reason Isaac repeats again the benediction he had previously pronounced was that the former one had been ineffective; given that, if he was a prophet and interpreter of the will of God, what had once proceeded from his mouth ought to have been firm and perpetual.

I answer, although the benediction was in itself efficacious, the faith of Jacob nevertheless required support of this kind. Just as the Lord, by frequently reiterating the same promises, detracts nothing either from Himself or from His word, but rather confirms the certainty of that word to His servants, so that their confidence would not at any time be shaken through the weakness of the flesh.

It must also be kept in mind what I have said: that Isaac prayed, not as a private person, but as one acting under a special command of God to transmit the covenant entrusted to him to his son Jacob. It was also of the greatest importance that Jacob should now, finally, be blessed by his father, knowingly and willingly, lest at a future time a doubt, arising from the recollection of his father’s mistake and his own fraud, might creep into his mind. Therefore Isaac, now purposely directing his words to his son Jacob, pronounces the blessing to be rightfully his, lest it should be thought that, having been previously deceived, he had uttered words in vain, by blessing someone under a false identity.