Recently I saw a question which had already been asked and is therefore a duplication. I will refer to these posts as new (the one posted most recent) and original post.
However, the original post had no answers 'marked as answer'.
Should this still be marked as a duplication.
The complication comes from when the original post was created - I understand that not all of those who post a question mark as an answer but if close a post as a duplicate then the likely hood of people seeing the original post is low (due to it's age) and an answer is less likely (assuming the original didn't have an answer).
We should direct people to the best thread for getting an answer.
The question should be well written, clear, and the most general case.
The answer(s) should be high quality, correct, understandable, and address the most cases or aspects.
Upvotes can be an indication of usefulness, but verify that they make sense. Extreme upvotes can mean that the question was in the Hot Network Questions, where high traffic and group think can result in voting that isn't really consistent with the actual post quality, or the relative quality compared to the other thread. On old threads, posts that are low quality (or even off-topic), by current standards were sometimes highly upvoted. Assess the actual posts.
An accepted answer, or lack thereof, doesn't necessarily mean much. Many question authors don't accept an answer even when there are several excellent ones. Many new users rush to accept an answer that may be crap. Acceptance reflects only the thought process of the questioner. Sometimes the accepted answer is a proven solution to the problem. Sometimes it is just the action of somebody whose opinion is that of a person who needed to ask the question because they were clueless.
Question age can be used if the threads are so similar in value that you would otherwise flip a coin to pick one. In that case, make the older one the primary. However, if you suspect that the answers in the old question might be out-of-date, see "Timing", below.
If there are multiple candidate questions to link to, weigh which has the best generic answers vs. which is most similar to the new question. You can propose one as a duplicate and comment about the other.
Take advantage of chaining. If one candidate is already closed as a duplicate of the other, linking to the closed question will also provide a path to the primary. Linking to the primary won't provide a path to the other.
Consider timing. If a new question is technically the same as one that was asked long ago, but the old question never received decent answers, or the answers might be dated, delay action on the duplicate. Give the new question a chance to attract current answers. Then decide which one to make the primary.
I'd say leave the newer one. Often people say things in different ways and some get through to people and some don't. If you delete the newer phrasing, it may never get answered (indeed, if the other Q is old enough, it might be more evidence that it wasn't phrased well for the group of people who might be able to answer it). Just as, often, there is more than one way of answering a question, there is also more than one way to phrase questions. Just because a particular phrasing doesn't resonate w/you, doesn't mean it wouldn't w/someone who knows the answer -- and I'd say this especially holds true for unanswered questions -- they are, perhaps, more likely to need rephrasing or reframing to get an answer (at least in my experience)...