What is the meaning of "Tov b'Ra O Ra b'Tov"?
Granted, the Torah must forbid swapping a blemished animal for a Tam. Why must it forbid the other direction?
Moshav Zekenim (citing Temurah 9a): "Tov" is repeated to limit Temurah to when it was initially Tov (Tam. If one was Makdish a Ba'al Mum, it does not get Kedushas ha'Guf, and cannot make Temurah.)
Why would it not suffice for the Torah to say 'Lo Yamir Oso b'Tov Oh Ra'?
Riva (3), Moshav Zekenim: It must write "Ra" twice, to teach that a Ba'al Mum makes Temurah (if it was Tam when it was Hukdash), and that a Ba'al Mum become Kadosh through Temurah (to forbid shearing or working with it).
Refer to 27:10:151:1,
There are two Lavim - "'Lo Yachalifenu v'Lo Yamir." Why does one who transgressed receive only 40 lashes (Temurah 2a)?
Temurah 4b: The Lav is Nitak l'Aseh ("v'Hayah Hu u'Semuraso Yihyeh Kodesh"). This exempts from the lashes for one of the Lavim.
Moshav Zekenim 1 : There is only one Lav. The Torah teaches which expressions make Temurah.
QUESTIONS ON RASHI
Rashi writes that all the more so, Tov b'Tov and Ra b'Ra are forbidden. We cannot punish based on a Kal v'Chomer!
Riva (3): Rashi said that it is forbidden due to a Kal v'Chomer. He did not say that one is lashed for it.
Moshav Zekenim: We can explain the verse to forbid [also] Tov b'Tov and Ra b'Ra; the first Tov limits Temurah to when it was initially Tov (refer to 27:10:151:1).