For the source text click/tap here: Chullin 113
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Shmuel explains that the repetition of the word "Gedi" in the verses of the prohibition against cooking meat with milk teaches that the Isur applies also to cooking Chelev with milk, and to cooking Neveilah with milk. The principle of "Ein Isur Chal Al Isur" does not apply, because Shmuel does not agree with that principle; he maintains that "Isur Chal Al Isur" (according to the Gemara's first version).
However, even according to the opinion that maintains "Ein Isur Chal Al Isur," it should be prohibited to cook Neveilah with milk. The Isur of meat and milk includes an Isur Hana'ah; it is prohibited to benefit from a mixture of meat and milk. Accordingly, the Isur of Basar b'Chalav is an "Isur Mosif" which adds an element of Isur to the existing Isur of Neveilah, because the Isur of Neveilah only prohibits the meat to be eaten, while the Isur of meat with milk prohibits deriving benefit from it.
We explore the galactic ramifications of this principle.