Rashi on Genesis 22:17/רש"י על בראשית כ״ב:י״ז

1ברך אברכך I WILL SURELY BLESS THEE — The double use of the term “bless” is intended to signify a blessing for the father and a blessing for the son (Genesis Rabbah 56:11).

2והרבה ארבה AND I WILL GREATLY MULTIPLY— once for the father, and once for the son (Genesis Rabbah 56:11).