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What to do with a newer post and a related, older, less thorough post?

Let me give a specific example. Today, I answered this post, and my answer seemed familiar. Looking through my old answers, I found that I'd answered this post a while back. The older post is a less thorough question, in the sense that answers to the older question won't completely answer the newer question, while answers to the newer question will be above-and-beyond answers for the older question.

Had they been posted in the other order, I'd mark the newer one as a duplicate without a second thought. I wonder what I ought to do in this case, though. I have three thoughts:

(1) Mark the older one for closing as a duplicate.

(2) Change my answer to the older question so that it actually answers the newer question, then mark the newer one for closing as a duplicate.

(3) Leave them both alone.

I think the last option is the least preferable, but I'm torn between the other two (though I'm leaning toward the first). Is there some consensus as to what should be done in such cases? Is there perhaps a better option that I'm not considering?

I think a consensus is important, here, because if there's enough disagreement, I worry that both of them will end up closed as duplicates of the other. To facilitate things, I've posted these three options below as answers. Give an upvote to whichever option(s) you favor. If you care to discuss you reasoning, put them as comments under your selection. If you've another option that you think is better, then please post it as an answer so we can all consider it. What say you?