The first is in Genesis 27, where the blessing is primarily about material abundance and position among his brothers, and among the nations:
Now may God give you of the dew of heaven,
And of the fatness of the earth,
And an abundance of grain and new wine;
May peoples serve you,
And nations bow down to you;
Be master of your brothers,
And may your mother’s sons bow down to you.
Cursed be those who curse you,
And blessed be those who bless you.”
Genesis 27:28-29 (NASB)
The second blessing is in Genesis 28, where Isaac pronounces God’s blessing over Jacob. It’s a much bigger blessing. It’s the IMPORTANT one.
May God Almighty bless you and make you fruitful and multiply you, so that you may become a multitude of peoples. May He also give you the blessing of Abraham, to you and to your descendants with you, so that you may possess the land where you live as a stranger, which God gave to Abraham.”
Genesis 28:3-4 (NASB)
Here, Isaac is not deceived. He knows exactly who he is giving this blessing to.
So I wonder if Rebekah’s intended deception was unnecessary. Perhaps she was so worried about material possessions and status of her son, she forgot that God had already promised to bless Jacob.
Perhaps Isaac was always going to give this bigger blessing to Jacob.