The origins of Elrond
Did you know that when Tolkien wrote The Hobbit, the character Elrond of Rivendell was not intended to be Elrond, son of Eärendil (who already existed in Tolkien's unpublished writings on the First Age)?
Elrond of Rivendell was conceived as a long-range descendant of Elrond, son of Eärendil, genetically in a position similar to that later occupied by Aragorn. His introduction in The Hobbit hints at this: The master of the house was an elf-friend -- one of those people whose fathers came into the strange stories before the beginning of History, the wars of the evil goblins and the elves and the first men in the North. In those days of our tale there were still some people who had both elves and heroes of the North for ancestors, and Elrond the master of the house was their chief.
When Tolkien was writing LotR and decided to bring back Elrond, he at first had the same idea in mind. In an early draft, Erestor is also described as being one of Elrond's kin and of mixed human and Elven ancestry; there was supposed to be a whole clan of them. Then it occurred to Tolkien that it would be really cool if Elrond of Rivendell actually was the son of Eärendil, still present in Middle-earth after all that time. Nothing he had written in The Hobbit ruled that out, so the two Elronds were merged and became the same person. Since Tolkien still wanted a clan of Men with distant Elven ancestry, Elrond developed a convenient twin brother to found that line.
This wasn't the only time Tolkien considered merging two characters with the same name, originally conceived as different people. The other famous case is that of Glorfindel: the one we meet in LotR borrowed the name of the hero of Gondolin, but was not intended to be the same person. Later, Tolkien again got the idea of making the two into the same character, in this case reembodied after death and returned to Middle-earth. The merging of the Glorfindels never made it into canonical publication, however; it came from late in Tolkien's life, and we know of it through an essay which appears in HoME.
~ Originally posted by Reera the Red