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.