To me, based on my experience on the site, there are a few reasons/situations where a question is marked as a duplicate. To me, I see these requirements being "either/or" requirements. If either of these are applicable, the "new" question is a possible duplicate.
To me, and your mileage may vary, the intentions of, and reasons for the question are 100% irrelevant. I don't care that you're just curious, or that your little daughter asked you a question, or even that you're doing homework; none of that makes a bit of difference as far as whether or not the question is valid or on-topic.
If none of the answers on the "original" question sufficiently answer the "new" question, then the "new" OP needs to explain why the questions are different and edit the "new" question to sufficiently differentiate itself from the "original".
I personally tend to try to boil questions down to what I feel is their real essence, or crux of the matter.
This makes a particular difference in situations like the Terminator vs Elephant question. To me, the fact that OP wanted to eventually judge whether or not the Terminator is able to push/pull/lift more than an elephant is irrelevant; to me, the crux of the question is "How much can a Terminator push/pull/lift?"
That's what's going on with the Vader/Obi-Wan questions, for me.
The question "Why did Obi-Wan leave Vader on Mustafar" boils down to "Why didn't Obi-Wan kill Vader or capture him?" The question has an answer. It has multiple answers. One of them is highly upvoted and has been accepted, but that is irrelevant here.
The question "Why didn't Obi-Wan try to end Vader's life to spare him from a horrible painful death by burning?" takes it from a slightly different angle, but boils down to "Why didn't Obi-Wan kill Vader out of mercy?"
I see no appreciable difference between asking why Obi-Wan did not kill/capture Vader, and asking why Obi-Wan didn't kill Vader out of mercy. Answering why Obi-Wan did not kill Vader is going to inherently include Obi-Wan let Vader suffer.
Even still, I initially refrained from voting to close. I wanted to allow the OP or anyone else to come back and try to explain why thought there was a difference. An hour or so after, despite continuing discussion, my opinion remained unchanged, so I went over the criteria/situation again.
- The question has been asked before, albeit with a different intention or reason for the question.
- The "original" question has an answer. Technically, the fact that there is an answer is all that is required. Someone has attempted to answer.
- The "original" question has an answer which does address the "mercy killing" angle. It is not the accepted answer, but that does not matter.
I was willing to hear reasons why the questions are distinctively different, or why the existing non-accepted answer was not sufficient, but the "new" OP never did that to my satisfaction.
So, I voted to close.