What does "Ya'an uv'Ya'an" mean? Why the double expression?
Rashi: It means 'because' (as a punishment for), 1 which the Torah repeats (presumably for emphasis).
Rashi (in the initial edition) and Rashbam: What the Torah is saying is that it is because Yisrael despised Hashem's judgments and because they loathed His statutes that the land became bereft of them.
Targum Onkelos and Targum Yonasan: 'Curses instead of blessings 2 will come upon you'.
Moshav Zekenim (25:17): It is due to an evil response to an Oni. Ya'an is an expression of answering, and uv'Ya'an are the letters of b'Oni.
Why does the Torah return to curse Yisrael after having written "v'ZachartiEs Brisi Yakov ... "?
Ramban: This refers to the period after Koresh initially granted Yisrael permission to return to Eretz Yisrael and ordered the rebuilding of the Beis-Hamikdash, when the order was withdrawn, and the land continued to be bereft of Yisrael and the land continued to be appeased for the broken Shemitos for another 19 years, until the second Beis-Hamikdash was built and they sanctified Yerushalayim with two Todos. 1 That was when the Kedushah returned to the land and they once again kept the Shemitah. 2
Moshav Zekenim: The exile and afflictions are a better Kaparah for Yisrael than Korbanos. It says regarding Korbanos "v'Nirtzeh Lo Lechaper Alav", and here it says "v'Hem Yirtzu Es Avonam" - all their sins. 3
Ramban: See Nechemyah, 10:32. In fact, the Ramban concludes, all that befell them during that period is hinted in this Parshah - as emerged from the explanations of the Ramban and the Seforno.
I would understand this if the verse said Avonoseihem. However, Avonam implies their one sin! (PF)