In 25:24 the passuk says Rivka gave birth "והנה תומים בבטנה" - behold there were twins in her belly.
We already know that she was carrying twins - Rivka, a few verses earlier went to Shem and Ever to ask what is going on because "The children struggled within her" and the explanation was "שני גיים בבטנך" - two nations are in your belly and will two nations will separate from you, and the older will serve the younger.
So we know already she is carrying twins, so what is the big surprise here, "Behold there were twins!"?
I think that no matter how much preparation a person does when he knows to anticipate difficult news or events, when it actually happens, when it actually comes to fruition, he will be surprised. he really, deep down, hopes that things will turn out better. He really has some internal denial that says maybe, just maybe, it will be normal.
Rivka must have believed what she had heard, but she was in denial. Deep down she hoped her baby would only be one, or at least would not split into 2 distinct nations that would battle each other.
When it actually happened, despite her foreknowledge, she was surprised to a certain extent.