Albert Barnes Commentary Numbers 18:15

"Everything that openeth the womb, of all flesh which they offer unto Jehovah, both of man and beast shall be thine: nevertheless the first-born of man shalt thou surely redeem, and the firstling of unclean beasts shalt thou redeem." — Numbers 18:15 (ASV)

Surely redeem ... redeem - A stronger expression is intentionally used in reference to the redemption of the first-born of man than in reference to that of unclean beasts. For the rule as to the former admitted of no exception: the owner of the latter, if unwilling to redeem, might destroy the beasts. Compare the marginal references.