English: “If a sheep” — why is this said (after “flock”)? To include the pesach for the fat-tail. “If a sheep” — to include a pesach whose time has passed, and shelamim that come by force of a pesach, for all the mitzvos of shelamim: semichah, libations, and waving of breast and thigh.
In Sifra: The specific word keshev (sheep), after the general “flock,” is not redundant. It pulls the pesach offering into the alyah rule and extends full shelamim ritual (leaning, libations, tenufah) to late or substitute pesach animals treated as shelamim.