לֹ֣א תֹאכְל֣וּ כָל־נְ֠בֵלָה לַגֵּ֨ר אֲשֶׁר־בִּשְׁעָרֶ֜יךָ תִּתְּנֶ֣נָּה וַאֲכָלָ֗הּ א֤וֹ מָכֹר֙ לְנָכְרִ֔י כִּ֣י עַ֤ם קָדוֹשׁ֙ אַתָּ֔ה לַיהוָ֖ה אֱלֹהֶ֑יךָ לֹֽא־תְבַשֵּׁ֥ל גְּדִ֖י בַּחֲלֵ֥ב אִמּֽוֹ׃ (פ)
너희는 너희 하나님 여호와의 성민이라 무릇 스스로 죽은 것은 먹지 말 것이니 그것을 성중에 우거하는 객에게 주어 먹게 하거나 이방인에게 팔아도 가하니라 너는 염소 새끼를 그 어미의 젖에 삶지 말지니라
Tze'enah Ure'enah
“You shall no boil a kid in its mother’s milk” [14:21]. You should not boil meat with milk. The Torah wrote three times that you should not boil meat with milk. This shows us the meaning of not boiling meat with milk. Even when you do not want to eat it, you would boil it for a gentile and the people were not warned about this. They boil meat with milk for gentiles in their homes. The second thing is, one should not eat meat with milk. The third is, one should have no benefit when one boils meat with milk, if one wanted to sell it to a gentile. He should not even give it to his dog.17Bahya, Deuteronomy, 14:21.
Rashi
לגר אשר בשעריך [YE SHALL NOT EAT ANY CARRION: THOU SHALT GIVE IT] UNTO THE STRANGER THAT IS IN THY GATES — i.e. unto a stranger that is a sojourner (גר תושב) — one who has undertaken not to worship idols (i.e. one who has been converted to the fundamental tenet of Judaism) but who eats carrion (does not obey the other teachings of the Torah) (Sifrei Devarim 104:2; cf. Rashi on Leviticus 25:35).
Chizkuni
לא תאכלו כל נבלה, “you must not eat the carcass of any animal that died of natural causes.” This includes carcasses of normally ritually pure mammals, free roaming beasts birds and ritually not pure fish. Ritually pure fish are permitted to be eaten whether they died from natural causes or were killed by man. The reason why this appears to have been repeated is so that no one would have an excuse to say that only the carcasses of mammals, free roaming beasts and pure birds are forbidden if they died from natural causes.