Why the Firstborn Donkey Is Redeemed With a Lamb

"And every firstling of a donkey you shall redeem with a lamb" (Exodus 13:13). Not with a calf, not with a wild animal, not with a slaughtered animal, not with a torn animal, and not with a hybrid. "And the firstling of a donkey you shall redeem with a lamb" - why is this said? Because Scripture says "yet you shall surely redeem" (the firstborn), I might hear that every firstborn of an unclean animal is included. Therefore the verse teaches, "and the firstling of a donkey you shall redeem with a lamb": you redeem the firstborn donkey, but you do not redeem the rest of the firstborn of an unclean animal. Or perhaps "the firstling of a donkey you shall redeem with a lamb" means you may redeem the rest of the firstborn of an unclean animal with garments and vessels? The verse teaches in another place (Exodus 34:20), "the firstling of a donkey you shall redeem with a lamb": the firstling of a donkey you redeem, but you do not redeem the firstborn of the rest of an unclean animal. If so, what does "yet you shall surely redeem" teach? Rather, if it does not refer to the matter of redeeming the firstborn of an unclean animal, apply it to the matter that one may consecrate an unclean animal for Temple upkeep and redeem it from that consecration. They raised a question: may one redeem with a ben pekua [an animal removed alive from its slaughtered mother's womb]? According to Rabbi Meir there is no question for you, since he says a ben pekua requires its own slaughtering, so it is a fully valid lamb. The question is according to the Sages, who say the slaughtering of its mother renders it permitted: what then? Since the Sages said the slaughtering of its mother renders it permitted, is it like mere meat in a basket? Or perhaps, since it runs about, we may still call it a lamb? Mar Zutra said: one may not redeem with it. Rav Ashi said: one may redeem with it. Rav Ashi said to Mar Zutra: what is your reasoning - that you derive "lamb, lamb" from the Passover offering? If so, just as there it must be a male, unblemished, and a year old, so too here it must be a male, unblemished, and a year old. And "you shall redeem, you shall redeem" comes to include more. But if "you shall redeem, you shall redeem" includes more, then even all kinds of things should be valid! If so, what purpose does "lamb, lamb" serve for you?