The Posuk says (Genesis 28:5):
וַיִּשְׁלַח יִצְחָק אֶת יַעֲקֹב וַיֵּלֶךְ פַּדֶּנָה אֲרָם אֶל לָבָן בֶּן בְּתוּאֵל הָאֲרַמִּי אֲחִי רִבְקָה אֵם יַעֲקֹב וְעֵשָׂו
To which Rashi comments on the fact that the Posuk says that Rivkah was both the Mother of Yackov and Eisav:
אם יעקב ועשו: איני יודע מה מלמדנו
Why does the Torah say it, and if he does not know the reason why does Rashi mention it at all?
Rashi was being intellectually honest. He knew that there was something to be learned from the redundancy, but he wasn't sure what it was. Being that Rashi got much of his material from midrashic sources, chances are he just wasn't able to find a midrash addressing this fact.
As to what the reason is behind the Torah mentioning that Rivka was the mother of Yaakov and Eisav, this is one of those things that you get to see the style of a commentary based on how they address this problem. Personally, I am most fluent in the commentary of Abarbanel, so I will give you his answer. Perhaps others can supplement with other answers.
The Torah here wants to emphasize the fact that Yaakov went to Aram, not because he was fleeing from Eisav's wrath, but rather just to find a wife. Thus, it shows that Yaakov was not travelling to a place where Eisav could not follow. Yaakov's relation to Lavan and family was equal to Eisav's, that is, he was:
לָבָן בֶּן בְּתוּאֵל הָאֲרַמִּי אֲחִי רִבְקָה אֵם יַעֲקֹב וְעֵשָׂו
So Eisav had just as much opportunity to go visit there as Yaakov did.
The Maskil LeDavid on the verse brings a fascinating explanation.
He says Rashi is explaining what Yitzchok was telling Yaakov. Yitzchok told Yaakov, I'm sending you to Lavan, but be careful, since I don't know what kind of man he is (i.e. Tzaddik or Rasha). Normally we can tell the nature of an uncle by looking at the majority of his nephews, but I have two sons, one righteous and one wicked, so there is no majority, and therefore it could go either way.
This then, is how to read Rashi: Rivka is the Mother of Yaakov and Eisav, and therefore I don't know what Lavan's nature is. This is what the Passuk is coming to teach us.
The Siftei Chachamim writes that Rashi was aware of the many interpretations by various commentaries, but he wasn't sure which one was correct according to the simple meaning of the verse.
The Lubavitcher Rebbe, based on his thesis that Rashi is only coming to explain the simple meaning of the verse (and only quotes Midrashim that can be used to explain the simple meaning), says (Likutei Sichot volume 5, pg 1 footnote 2) that when Rashi says, "I don't know", it isn't because he is not aware of any of the answers given, but rather is not aware of any answer that explains the simple meaning of the verse.
This also explains why there are some instances where Rashi gives an explanation in his commentary on the Talmud, but says I don't know in his commentary on the Torah.
[See there for a list of locations where Rashi says "I don't know" or something similar, including places where Rashi says "I don't know" and then proceeds to give a reason.]
Rashi is studied by trying first to understand "what is bothering Rashi?". This means that every perush was motivated by a question he had or something that was not completely clear. There are some situations, like this one, in which Rashi felt he couldn't provide the answer. But still there is a question and he wanted to bring it before us.
This shows us a great lesson always be humble........ like Rashi :-)
I heard from a rav who explained that any time Rash"i says, "I don't know" it means that he researched several sources and found more than one that sounds plausible. But, he doesn't know which opinion is the correct or best explanation.
