I believe that Glorfindel in the Sil was not the same person as Glorifindel in the Lotr: the Sil states plainly:
I think Glorfindel in the Lotr was of the same house as Glorfindel of the Sil, and possibly named after him for his physical resemblance. At any rate, I agree that Glorfindel was of Vanya descent; not fully Vanya, as none of them ever returned, but maybe a brother-in-law of Turgon's.Many are the songs that have been sung of the duel of Glorfindel with the Balrog upon a pinnacle of rock in that high place; and both fell to ruin in the abyss.

