In the Silmarillion, an elf named Glorfindel is killed in a duel with a Balrog at the battle for Gondolin. (He kills the Balrog as well.) Then in The Fellowship of the Ring, an elf named Glorfindel meets Frodo, Sam, Merry, Pippin and Aragorn and helps them get a wounded Frodo to Rivendell.
This would be the only time that Tolkien uses the same name for two different elves. Is there any connection between these two elves?
Glorfindel is confirmed as the same Elf in two separate passages in the History of Middle-earth (HoME).
First of all from The Return of the Shadow (Vol VI of HoME), written by Christopher Tolkien:
Also very notable is 'Glorfindel tells of his ancestry in Gondolin.' Years later, long after the publication ofThe Lord of the Rings, my father gave a great deal of thought to the matter of Glorfindel, and at that time he wrote: '[The use of Glorfindel] in The Lord of the Rings is one of the cases of the somewhat random use of the names found in the older legends, now referred to as The Silmarillion, which escaped reconsideration in the final published form of The Lord of the Rings.' He came to the conclusion that Glorfindel of Gondolin, who fell to his death in combat with a Balrog after the sack of the city (II.192 - 4, IV.145), and Glorfindel of Rivendell were one and the same: he was released from Mandos and returned to Middle-earth in the Second Age.
Secondly, an essay from the People of Middle-earth (Vol XII of HoME) in Tolkien's own words detail how Glorfindel would be able return from death:
When Glorfindel was slain his spirit would then go to Mandos and be judged, and then would remain in the Halls of Waiting until Manwe granted him release. The Elves were destined to be by nature 'immortal', within the unknown limits of the life of the Earth as a habitable realm, and their disembodiment was a grievous thing. It was the duty, therefore, of the Valar to restore them, if they were slain, to incarnate life, if they desired it - unless for some grave (and rare) reason: such as deeds of great evil, or any works of malice of which they remained obdurately unrepentant.
When they were re-embodied they could remain in Valinor, or return to Middle-earth if their home had been there. We can therefore reasonably suppose that Glorfindel, after the purging or forgiveness of his part in the rebellion of the Noldor, was released from Mandos and became himself again, but remained in the Blessed Realm - for Gondolin was destroyed and all or most of his kin had perished.
Tolkien has a couple of theories as to when Glorfindel returned:
We could then reasonably suppose that Glorfindel (possibly as one of a small party, more probably as a sole companion) landed with Gandalf - Olorin about Third Age 1000.
Alternatively:
We may then best suppose that Glorfindel returned during the Second Age, before the 'shadow' fell on Numenor, and while the Numenoreans were welcomed by the Eldar as powerful allies.
It depends a bit on your point of view. Glorfindel died and was eventually reincarnated, essentially the same as Gandalf was (though, being a maia, Gandalf's revival required the intervention of Eru, whereas the Valar did it themselves for Glorfindel). So it is the same Glorfindel in spirit, but not in body, however you wish to interpret that.
According to the LotR wiki, In the First Age:
[Glorfindel] slew a balrog, though he died in the process. Like all elves, he was re-embodied in the Halls of Awaiting.
Then in the Third Age,
In recognition of his skill, he was sent by the Valar back to Middle-earth either as a precursor to or companion of the Istari to aid the fight against Sauron.
And later in the article:
He was said to have been resurrected by the Valar and be given the same degree of power as a Maiar.
And finally (and perhaps most directly related to the question at hand):
There has been some confusion about the Glorfindel of Gondolin and the Glorfindel of Rivendell. They may or may not have been meant to be the same person (though an early draft of The Fellowship of the Ring contained a note that "Glorfindel tells of his ancestry in Gondolin"). However, as Tolkien felt that elf names were unique (though he doubled others), he decided to find a way to correct his perceived mistake. He did this by making them the same but reincarnated person. Tolkien stated that Glorfindel's spirit returned to the Halls of Awaiting, but was after a time re-embodied by the Valar. He then returned to Middle-earth (either in the mid-Second Age, or as a companion of the Blue Wizards in the Third).
He is also said to be an Elven Prince, which has led to speculation that Glorfindel is the son of Finarfin or one of his descendants.
So it appears it was originally a mistake, but JRRT managed to find a way to correct it.
It's not entirely clear, and I can direct you to further discussion here. I personally prefer the reading that Glorfindel is in fact a re-embodiment of the Glorfindel of Gondolin, giving us a glimpse of the relationship between the Elves of ME and the fantastic powers beyond the sea. It's clear in several examples that it is within the power of the Valar to grant re-embodiment where it is desired and especially deserved, as with Gandalf and with Beren and Luthien.
Besides that, Elves are long-lived (immortal) - I'm pretty sure there's no other example of Elves sharing the same names (though Men have been named after both Men and Elves).
