In fact, the question of the two Glorfindels was noted by Tolkien himself, who attempted to resolve this question (see The History of Middle-Earth, Volume 12) He concluded that they were in fact the same person, the Glorfindel of Rivendell being a reincarnation (by way of the Halls of Mandos) of Glorfindel of Gondolin.

However, since this was a private note, not published work (at least during Tolkien's lifetime), many consider this question unresolved.