[doug*platypus]

I think that the habit of The Professor to repeat ideas was so that they could appear in published form in The Lord of the Rings. If I'm not mistaken, much of the material presented in The Silmarillion was written at an early date, and Tolkien never had more than a faint hope that it would be released.

I think that the ideas, characters and devices used in the tales of the First Age were so powerful that they had to be brought out into the open. Thus the story of Aragorn and Arwen, as well as being a great tale in itself, was a homage to the story of Beren and Lúthien (hence Aragorn being the one to relate part of the tale to us). The story of an immortal being giving up their right to an endless existence, and even the right to wait in the Halls of Mandos if they did die (remember Arwen could have been reunited with her mother in this way), so that they could love a mortal, is a very strong story.

Likewise, the battle between Gandalf and Durin's Bane has not only the same structure as Glorfindel's death, but the same themes. Gandalf gives his life to save the Fellowship (as Glorfindel dies to protect the refugees of Gondolin), and eventually is rewarded for his sacrifice by being allowed to return to Middle-Earth to continue the fight against Sauron.

I'm not sure this is a case of history repeating itself, but themes in Tolkien's writing repeating themselves.