"And it killed a man or a woman" — this phrase appears in the mued section, but the Mekhilta says it is "extra." Its legal content is already known from other verses. So why is it here? To serve as the basis for a gezeirah shavah — a verbal analogy between the tam and mued sections.
The Mekhilta then addresses why "the ox shall be stoned" is repeated in the mued passage when it was already stated for the tam. Can it not be derived logically? If a tam — which gored for the first time — is stoned, then surely a mued — a known repeat offender — should also be stoned. The a fortiori argument seems airtight.
But the Mekhilta raises an objection. Perhaps the tam is stoned precisely because its owner does not pay kofer. The stoning is the full punishment. For the mued, where the owner does pay kofer, perhaps the stoning can be waived — the monetary payment substitutes for the animal's death.
The Torah forecloses this possibility by explicitly repeating "the ox shall be stoned" in the mued section. Both financial payment by the owner and stoning of the ox are required. One does not replace the other. The kofer is paid to the victim's family. The ox is stoned as a matter of justice. Two consequences, two purposes, both mandatory. The Torah does not allow you to buy your way out of the ox's death sentence.