Chullin 132 - Gifts for the Daughter of a Kohen
Ulla used to give kohen's gifts to a kohenet - a daughter of a kohen. Why? When the Torah told to give the gifts to a kohen, we understand that a kohenet is excluded. However, when it said the word "kohen" again, this is a double exclusion, which always means an inclusion, and a kohenet is now included. Rav Pappa was married to a kohenet, and people would bring him kohen's gifts.
A first male of kosher animal is normally given to a kohen. If one firstborn became mingled with a hundred animals belonging to different people, and each owner slaughters his animal, nobody gives kohen's gifts. Each owner can claim that it is he who is slaughtering the firstborn, and kohen's gifts are not applicable to his animal. Even though the majority of the animals are obligated in kohen's gifts, in money matters we do not follow the majority, but the prevailing rule is that one who wants to extract money from his fellow needs to bring a proof to his claim, and here the kohen looses. However, if one person slaughters all of the animals, he needs to give kohen's gifts from all but one animal.
Art: Eugène Verboeckhoven - A Panoramic Summer Landscape With Cattle Grazing In A Meadow By A Windmill