What is the significance of the double expression, "Ra'o Ra'inu"?
Rashi: 'We saw in your father, and (now) we see in you.' 1
Avimelech might also have been referring to the numerous miraculous miracles that they witnessed, as described in the course of the last few Pesukim. (EC)
Perhaps it means that not only did HaSh-m bless Yitzchak while he lived with them, but he also punished them after he left them. 2 (EC)
Ohr ha'Chayim #1: We saw that we cannot rely on Avraham's Shevu'ah [for it can be permitted]. Also, we saw that we were not meticulous about the details. 3 Therefore, we came to make a new Bris with you.
Ohr ha'Chayim #2: We saw that HaSh-m is with you wherever you go, so our hatred ceased. We saw (i.e. understood) that the existing Shevu'ah does not suffice (refer to 26:28:1:4*).
Malbim, Ha'amek Davar: Initially, we thought that your success is due to [our] land, and you diminish our abundance. Now [that you prosper also elsewhere,] we see that [you succeed because] HaSh-m is with you [and we were blessed due to you]!
This is particularly relevant in light of the fact that Yitzchak had only now come into his own (Refer to 26:25:1:1), on account of which they now placed Yitzchak on a par with his father. This also explains why they had now come to renew the covenant that they had made with Avraham, with Yitzchak.
Ohr ha'Chayim: One who swears to extortionists or murderers may nullify in his heart (Nedarim 28a). Since we had power over Avraham, he could have annulled. Similarly, only now that you are not living among us, we can properly ask you to swear. This is why we sent you away! What we told you "Ki Atazmta Mimenu" was not the primary reason.
What is meant by the seemingly repetitious, "Alah Beinoseinu, Beineinu u'Veinecha"?
Rashi: The curse that was between us from the days of your father, should be between us and you now.
Riva: We will swear in the name of the gods common among us (our idolatry).
Ohr ha'Chayim #1: They needed to add a Bris with Yitzchak to include all his seed, for only part of Yitzchak (i.e. Yaakov) is considered Zera Avraham. 1 This Bris will include even Esav; also he is Zera Yitzchak.
Ohr ha'Chayim #2, Malbim #2: It also upholds the Bris with Avraham.
Ohr ha'Chayim #3: Beinoseinu means that it is with all of us. It is Al Da'as Rabim, so it cannot be permitted.
Malbim #1: You will be like my firstborn. This is like a Bris between brothers.
Ha'amek Davar: We will add an Alah (curse), for only a Yarei Shamayim is concerned for a Shevu'ah. It is also with the nobles, who annulled the first Bris due to their envy of you.
This is astounding, that Avimelech heard and understood "b'Yitzchak Yikarei Lecha Zara" (21:12)! (PF)
QUESTIONS ON RASHI
Rashi writes: "Let there be an oath between us [ - between us and you] - May the oath that [already] exists between us since the time of your father (Avraham), be [renewed] between us and you now also." Why explain in this way?
Gur Aryeh: The wording in the verse is redundant; it would have sufficed to write, '... an oath between us.' Rather, the first phrase refers to the oath Avraham had already made. The verse means, "May the oath [that already exists] between us [since Avraham], now be renewed between us and you [as well]."