We shouldn't leave it alone because then it leaves two different versions of the same thing. Accepting doesn't really mean anything, except that the OP said that the solution worked, and they didn't need any more help. Even if it's accepted, there may be other, better answers to the question that still can be submitted. That may not sound like a big deal with one duplicate question, but if it's a hot topic that has 5-10 different questions that are exactly the same, some might be ignored, and it just adds clutter to the site.
A good analogy (involving Ubuntu :D) would be not submitting a pull request for a patch of Ubuntu, just making people download the patch
If there's a good answer, flag it, and request that a moderator merges the two posts to move the answer(s) to the first thread.