When a "Lav" is repeated in the Torah

I have to warn you on this one that in order to understand it you really have to look at the gemara in Pesachim on 24a. If I am mistaken in anything, please feel free to correct me.

In Shoresh 9 of Sefer HaMitzvos and also in Lav 179 the Rambam says that if a Lav is repeated in the Torah you do not get two sets of Malkos and it isn't counted twice in the minyan hamitzvos. The Rambam explains the sugya in Pesachim on 24a that one gets 4 sets of malkos if he eats a potisa not because of kefel halavin, but rather because a potisa has 4 different sheimos, it counts as 4 different "types" of bugs that the Torah says are prohibited (it's a land bug, water bug etc.). Other Rishonim argue with the Rambam and say that a potisa is water bug and you get 4 different types of malkos because it is a lav that is repeated 4 different times. (For more discussion of this see the Rambam in Maachalos Asuros 2:23 and the Raavad there.
The Ramban (in shoresh 9) asks on the Rambam from the gemara in Pesachim 24a that compares eating a potisa to eating Kodshim B'Tumah where it would seem to be a situation of kefel halavin, a repetition of lavin.
The Gri"z (maachalos asuros 2:23) answers that by eating kodshim b'tumah there are also two shemos, #1 eating korbanos b'tumah and #2 eating kodshim b'tumah. The Gri''z brings several proofs to the existence of these two separate dinim.
The problem that I had with this is that the Rambam himself in Shoresh 9 uses this gemara about eating kodshim b'tumaah to prove his points regarding kefel halaavin. The proof is that the gemara with regards to eating kodshim b'tumaah calls it laavei yesarai, "extra" laavin. So the Rambam proves from this language that they are indeed extra and don't count in minyan hamitzvos. Thus, even though the Gri"z gains an understanding in how the gemara could compare eating a potisa to eating kodshim b'tumaah, still the response of Rav Ashi in that sugya of "laavei yeseiri" is no longer a proof to the Rambam, which the Rambam himself brings.