Why is Korach mentioned twice, once here together with the children of Adah, and again in 36:18, together with the children of Aholivamah?
Rashi (in Sotah) and Ramban: There were two Korachs in the family of Esav. 1
Ramban (citing the Ibn Ezra): Korach was the youngest son of Aholivamah. After her death, Adah raised him.
Rashi (to 36:5): Korach was a Mamzer; he was the product of Elifaz and Aholivamah, his father's wife.
Refer to 36:16:0.2:1*.
Why does the Torah list Korach, who was a son of Aholivamah, among the sons of Elifaz?
Rashi (in Sotah): The Korach mentioned here was a different one, a son of Elifaz. 1
Peninim mi'Shulchan ha'Gra: After Elifaz married Timna (his concubine), he changed the name of his son Timna to Korach. 2
Ha'amek Davar: Elifaz had a son Timna. To avoid confusion with his concubine, his son Timna was called by his son's name (Korach).
Malbim (to Divrei ha'Yamim I 1:36): Elifaz fathered Korach through his father's wife Aholivamah. His mother called him Korach, and Elifaz called him Timna, like his concubine. Initially Timna is not listed among Elifaz' sons, for people thought that he was Esav's son. There is a mere hint; "Timna" can be read with both verses, i.e. Elifaz also had a son Timna (refer to 36:12:2:4 and the note there.) When the Torah lists chiefs of Elifaz, he is listed there with the name Korach; at that time it was known that he was really Elifaz' son. 3 To teach us this, Ezra listed (Divrei ha'Yamim I 35,36) Korach among Bnei Esav, and Timna among Bnei Elifaz.
Even though, for some reason, the Torah did not initially list him there. In fact, Rashi in Sotah (Daf 13a) says this to explain why the Torah lists 'Aluf Korach' twice.
This explains why the Pasuk lists Korach among the chiefs of Elifaz, but does not list Timna.
How did this become known? Even if they knew that he was not from Esav (e.g. Esav was not intimate with her in the months preceding her pregnancy, or he had lost the ability to father children), and Elifaz and Aholivamah admitted to the adultery, perhaps she had Zenus also with others! Perhaps Korach looked exactly like Elifaz (refer to 25:19:2:2, and to Devarim 32:18:1:4). (PF)
Why does it say "b'Eretz Edom" regarding the chiefs of Elifaz and Re'u'el, but not for those of Bnei Aholivamah?
Ha'amek Davar: The chiefs of Elifaz and Re'u'el were in Eretz Edom only a short time, and then they moved away, but they were not called Edom. Bnei Aholivamah are the primary Edom in Har Se'ir, for they are from Se'ir ha'Chori (she was his granddaughter or great granddaughter).
