Ultimately, you should always point users the "best" possible information you have. The wiki nature of Stack Exchange exists so you can constantly improve your content. Ideally, you would have caught the duplicate post before it split the conversation, but if a better (duplicate) question went on long enough to attract demonstrably better answers, you should make that THE canonical source of that information.
But remember that you have a lot more tools are your disposal that simply closing one question or the other. If both threads contain great information, you can also consider merging the answers. And if the new question was simply clearer, consider editing the original to incorporate the qualities of whatever you found more intriguing in the second post.
Recall also that we changed the [closed as duplicate] description to say, "this question already has an answer elsewhere…" rather than focusing on the question itself. Closing a question as duplicate provides another entry point help folks searching for that information, but we're here to help people find the best possible answer rather than worrying about who asked the question first. You should always look at the entire thread to decide which should be the canonical source of that information. Sure, if the quality of both questions happens to be a dead heat, you should stick to the original thread… but generally speaking, you should always point them to the best possible information you have.
Essentially, you should never favor the lesser-quality thread because it was "posted first."