Lucas definitely had sequels in mind, but it was very much in doubt whether the movie would succeed. There was an official sequel novel by Alan Dean Foster called Splinter of the Mind's Eye released in '78, which was explicitly designed to be filmable on a very low budget if the original didn't do too well. That does feature Luke duelling Vader with his saber.

It also (rather uncomfortably in light of later revelations) features Luke & Leia as a romantic couple! Apparently Lucas signed off on Foster's manuscript, so he hadn't had the idea that they were siblings yet.

Luke and Leia were teased as a romantic couple in Empire, she kissed him on the lips to draw Han's ire, and then it's hinted that she's his sister in the final scene of Dagobah/when he calls out to her via the force. The incest thing is something that Lucas was deliberately playing with, whether Splinter took that into account or not.

How do you get “deliberately” from that?

The comment you’re replying to is correct; there’s plenty of evidence that Lucas made up both the plots and crucial details of all the Star Wars films as he went along, rather than plotting it all out in advance. Telling people he had, or at least allowing them to believe so, was just another inspired marketing trick.