[Bruce MacCulloch]

On the contrary, the books do say that the two Glorfindels are the same person.
Regarding Glorfindel, Tolkien said:

An Elf who had once known Middle-earth and had fought in the long wars against Melkor would be an eminently suitable companion for Gandalf. 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. This supposition would indeed explain the air of special power and sanctity that surrounds Glorfindel - note how the Witch-king flies from him, although all others(such as King Earnur) however brave could not induce their horses to face him (Appendix A (I, iv), RK p. 331). For according to accounts (quite independent of this case) elsewhere given of Elvish nature, and their relations with the Valar, 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.

HoME, vol 12, The Peoples of Middle Earth
Also, in the same book, Professor Tolkien has this to say regarding the identity of Glorfindel:

At any rate what at first sight may seem the simplest solution must be abandoned: sc. that we have merely a reduplication of names, and that Glorfindel of Gondolin and Glorfindel of Rivendell were different persons. This repetition of so striking a name, though possible, would not be credible. No other major character in the Elvish legends as reported in The Silmarillion and The Lord of the Rings has a name borne by another Elvish person of importance. Also it may be found that acceptance of the identity of Glorfindel of old and of the Third Age will actually explain what is said of him and improve the story.

Tolkien also states that another possibility is that Glorfindel came back some time during the Second Age - still indicating that there are not two Glorfindels, but one.

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. His return must have been for the purpose of strengthening Gilgalad and Elrond, when the growing evil of the intentions of Sauron were at last perceived by them.

The only possible conclusion to draw is that Glorfindel of Gondolin and Glorfindel of Rivendell were one and the same. To me, it seems rather specifically stated.

[ February 19, 2002: Message edited by: Bruce MacCulloch ]