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 Eisav. 1
Rashi (to 36:5): Korach was a Mamzer; he was the product of Elifaz and Aholivamah, his father's wife.
Ramban (citing the Ibn Ezra): Korach was the youngest son of Aholivamah. After her death, Adah raised him.
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 Eisav'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 Eisav, 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 Eisav (e.g. Eisav 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 the Torah insert "be'Eretz Edom" in connection with the chiefs of Elifaz and Re'u'el, but not with those of the sons of Aholivamah?
Ha'amek Davar: Because, whereas the chiefs of Elifaz and Re'u'el were in Eretz Edom only a short time before they moved away, and were therefore not called ?Edom?, the sons of Aholivamah were the primary Edom in Har Se'ir, seeing as they were from Se'ir ha'Chori (who was her grandfather or great grandfather).
