I had believed that the two Glorfindels were always different and that possibly
the one from the LoTR was named after the one from
Gondolin. This was for two reasons:
1) The two Glorfindels act very differently from each other. First of all, the
Glorfindel of Gondolin was a great warrior, one of the key generals of
Turgon, which meant he must have been very great, and he also killed a
Balrog in battle, something that even
Gandalf had difficulty doing. However, the Glorfindel of LoTR, although he does manage to confront all nine of the
ringwraiths, is not given nearly the same importance as the Glorfindel of Gondolin.
2) There are many cases of people and
places being named after others.
Lothlorien is named after the garden of
Lorien in
Valinor.
Denethor is named after the
elf of
Ossiriand. Mablung of
Ithilien is named after Mablung of
Doriath.
Ecthelion of
Gondor is named after Echtelion of Gondolin. So I assumed that Glorfindel was such a case.
However if JRRT said so, it must be right...I guess, although I have a hard time imagining it. I heard that Glorfindel, like
Beren, was released by
Mandos, on another thread somewhere. I don't remember...