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). Consequently, the Torah needs to issue the warning many times.
Hadar Zekenim: Because whereas the earlier Pasuk forbids tilting his verdict in judgment, the current Pasuk is discussing [oppressing him financially] even not in judgment. 1
Oznayim la'Torah: The earlier Pasuk is referring to all of Yisrael, whereas the current Pasuk is speaking to the Dayan. 2
Hadar Zekenim: That is why the Torah follows with the command to leave one's field in Sh'mitah for the poor to eat, incorporating Geirim.
Oznayim la'Torah: Which explains why it omits "Lo Soneh" (Do not taunt) since it it is not the way of Dayanim in Beis-Din to say to a litigant 'Remember the deeds of your father!'.