Why does the Torah warn against oppressing a Ger so many times (See for example, above, in 22:20)?
Rashi: Because 'Suro Ra', which means, either that he has a strong Yeitzer ha'Ra and is easily led to sin, or that, once he strays from the path of Torah, it is hard for him to return. (Sifsei Chachamim).
Hadar Zekenim: Above the Torah forbids tilting his verdict in judgment, and here, [monetary affliction] even not in judgment. It then says to leave your field in Sh'mitah for the poor to eat, and Gerim are included.
Oznayim la'Torah: The earlier Pasuk is referring to all of Yisrael, whereas the current Pasuk is speaking to the Dayan. 1
Oznayim la'Torah: Which explains why it omits "Lo Soneh" (Do not taunt) since it would not be the way of Dayanim in Beis-Din to say to a litigant 'Remember the deeds of your father!'.