There are a number of oddities in this genealogy, which makes me wonder if the boringness of these genealogies sometimes caused the scribes to be a bit more lax in copying them than in other places. Before we even get to Bashemath, there's the matter of Anah, which the SAB interestingly misses. There may be two people here named Anah, but if so, it's an uncle/niece, and I'm finding it hard to believe that someone would name their daughter after their brother even in modern times, much less in the ancient Middle East. In this chapter, Anah is referred to as: the parent of Aholibamah (vv. 2, 14, 18, 25), a "daughter" of Zibeon (vv. 2, 14), a son of Zibeon (v. 24), a son of Seir (v. 20), a Horite "duke" (v. 29). The only other place that the name Anah is mentioned is in 1Chronicles, where we also see mention of a son of Seir and a son of Zibeon by this name, which might lend credence to the idea that there were two of them, but I strongly suspect that the "daughter" references at least are a mistake in gender. (In verse 24, the translation "mules" is a bit suspect. In Robert Alter's translation of Genesis, he assumes that the Hebrew word "yemim" is an accidental transposition of the letters of "mayim", which means "waters". Finding water in a desert wilderness is something perhaps more noteworthy than finding mules.)
Another minor matter that I noticed in trying to sort out the names in this genealogy was that Korah was listed as a son of Eliphaz in verse 16, which seems to also be a mistake, as the only other use of that name in this chapter is in reference to a half-brother of Eliphaz. Sure, it's possible that Eliphaz had a son named Korah that the writer forgot to mention earlier in the chapter, but I'm inclined to see this as a copyist error somewhere. Oh, and then there's the issue of whether these people are Hittites, Hivites, or Horites; it appears that may be largely interchangeable terms.
Okay, Bashemath's father... I ducked this one before, let me take another look. I swear that the SAB had a page dedicated to the subject of Esau's wives, because this is indeed a touchy subject, and I know I read about it somewhere on the net. I thought it was there. Verses in question:
26:34 And Esau was forty years old when he took to wife Judith the daughter of Beeri the Hittite, and Bashemath the daughter of Elon the Hittite:It really seems that something is very mixed up here. I suppose it might be possible that Esau had more than three wives, although it would be very odd not only to have six wives, but to have among those six two sets of sisters and two wives with the same name. There may be an explanation for all of this, but I don't know what it might be.
28:9 Then went Esau unto Ishmael, and took unto the wives which he had Mahalath the daughter of Ishmael Abraham's son, the sister of Nebajoth, to be his wife.
36:2-3 Esau took his wives of the daughters of Canaan; Adah the daughter of Elon the Hittite, and Aholibamah the daughter of Anah the daughter of Zibeon the Hivite; and Bashemath Ishmael's daughter, sister of Nebajoth.
Basically, I'm saying despite the fact I've addressed the issues of polygamy and the Amalekites, I don't think I can explain most of this chapter, as it's one of the stranger genealogies in the Bible in my opinion.