After Yaakov was blessed by his father, Yitzchak, it says (Bereishis 27:30): “Vayehi ach yatzo yatza Yaakov me’eis pnei Yitzchak.” The redundant words יצא יצא come to teach us that when Yitzchak blessed Yaakov, Rivka answered amen to his brachos, as amen is numerically equivalent to א“צ. The reason that the Torah said יצא יצא twice is to teach us that Rivka answered amen to each one of the two pesukim of the brachos of Yaakov, the same way we answer amen after each one of the three pesukim of Birchas Kohanim. Therefore, even after Yitzchak discovered his mistake, he said about Yaakov (passuk 33) “gam baruch yihiyeh” – because he knew that a brachah that is answered by amen will be fulfilled.
For this reason, even when Rivka quoted to Yaakov what Yitzchak told Eisav, she said (Bereishis 27:6-7): “I have heard your father speaking to Eisav your brother, saying, bring me game and make me tasty foods, and I will eat, and I will bless you before the Hashem before my death.” Rivka added the words “lifnei Hashem”, which Yitzchak didn’t say, because she knew that when she would answer amen to his brachah, the brachach would be considered to be “lifnei Hashem,” as it is known that amen is numerically equivalent to the Names of Havay-ah and Adnus. (Chayei Yitzchak Parashas Toldos)
