[Aiwendil]

I agree that the two statements about Taur-nu-Fuin are redundant, but it's not immediately obvious to me which should be kept and which removed. I suppose that GA being the later text, it should be the second, despite the fact that the QS description is more vivid. Of course, we could combine them, but this risks awkwardness. I think I would advise:

RB-DB-19.1: But Morgoth pursued his people to the death, until few remained; and he took all the forest and the highland of Dorthonion, save the highest and inmost region, and turned it little by little to a place of such dread and lurking evil that even the Orcs would not enter it unless need drove them. {Therefore it was after called by the {Gnomes}[Elves] {Taur-na-Fuin}[Taur-nu-Fuin], which is Mirkwood, and Deluwaith, Deadly Nightshade; for the trees that grew there after the burning were black and grim, and their roots were tangled, groping in the dark like claws; and those who strayed among them became lost and blind, and were strangled or pursued to madness by phantoms of terror.}

RB-DF-09: <GA {but}But {[struck out: still]} Barahir would not retreat and defended still the remant of his land and folk in Dorthonion. But Morgoth hunted down all that there remained of Elves or Men, and he sent Sauron against them; and all the forest of the northward slopes of that land was turned into a region of dread and dark enchantment, so that it was after called Taur-nu-Fuin, {the Forest under Nightshade}<QS which is Mirkwood, and Deluwaith, Deadly Nightshade>.

Thus we keep both names. An alternative would be to transfer some of the description as well:

RB-DF-09: <GA {but}But {[struck out: still]} Barahir would not retreat and defended still the remant of his land and folk in Dorthonion. But Morgoth hunted down all that there remained of Elves or Men, and he sent Sauron against them; and all the forest of the northward slopes of that land was turned into a region of dread and dark enchantment, so that it was after called Taur-nu-Fuin, {the Forest under Nightshade}<QS which is Mirkwood, and Deluwaith, Deadly Nightshade; for the trees that grew there after the burning were black and grim, and their roots were tangled, groping in the dark like claws; and those who strayed among them became lost and blind, and were strangled or pursued to madness by phantoms of terror>.