Whatever path they took to eventually land on the most famous twist in cinematic history, the idea was not original to Lucas or Kurtz. It almost certainly was inspired by the same twist in DUNE, the novel, where Paul Atreides finds out he is the grandson of the primary villain, Vladimir Harkonnen. And so many other aspects of Star Wars seem lifted or inspired by Dune, I think it’s fairly obvious at this point, if Lucas was wrestling with anything about Vader’s backstory, it was how much ‘on the nose’ he could be without looking like he actually ripped it off directly from Dune.
Well, the dropped story idea* that’s attributed to a Kurtz quote (where Kurtz says that Lucas told him this circa late 1975) sounds nothing like the Dune subplot. So there’s that.
*Annikin/Anakin/Luke’s father sometime during the back-story kills Darth Vader/Sith Knight then takes on Darth’s identity, more of a ‘Darth is not who you think he is’ story. Then the sequel (“book two”) would have revealed who he really was. Presumably Vader wouldn’t have necessarily himself revealed his identity to Luke like he does in ESB - perhaps a still in the flesh Ben Kenobi would have discovered the truth in the second one.