The Context

On the seemingly repetitious opening verse of parshas Toldos, “And these are the generations of Yitzchak the son of Abraham; Avraham begat Isaac,” Rashi cites a Midrashic teaching:

“The scorners of the generation were saying that Sarah had conceived from Abimelech, for she had lived with Avraham for many years and had not conceived from him. What did G-d do? He shaped the features of Yitzchak’s face to resemble Avraham’s, and everyone attested that Avraham had begotten Yitzchak. This is the meaning of what is written here: “Yitzchak, the son of Avraham,” because here is proof that “Avraham begat Yitzchak.””