I think in some funny way, Obi-Wan lost in his first duel. Victory wise, he was the winner. But he lost something in the process with having to at the time, kill his friend.
One the last round, Kenobi won.
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Was it really a negative that he didn't kill Vader? Had he killed Vader, The Emperor would have just looked for a new apprentice faster and found a much more younger and powerful apprentice. Part of the problem between Sidious and Vader was that Sidious was frustrated that Vader was turned into shadow of his former self just as soon as Sidious turned him. He would've has spent more time looking for a new apprentice had Vader been killed on Mustafar.