The following are my opinions, and may or may not be representative of the will of the community or moderation team:
"the original question's Accepted answer, if there is one, is announced as being the answer which also clarifies the issue for the [poster of the duplicate question] [...] I see this as slightly problematic"
I think you're correct that the duplicate poster does end up losing their right to indicate their accepted answer, and that this is perhaps not ideal. But I would also tend to argue that it's more important for the original poster to have the right to accept an arbitrary answer on their question than it is for a user who posts a duplicate to get that same right on somebody else's original question. I don't think overriding an original question's accepted answer is the way to go. But should the dupe poster retain any input on their answer preference?
I acknowledge that asking original questions can be difficult; Stack Exchange is a large place, and even after careful searching it's still possible to unwittingly post something that's been answered or almost completely covered before.
Would it be possible and worthwhile to find a way to allow posters of duplicate questions to indicate the answer they would accept from the original? Keeping in mind that anyone finding their question marked as a dupe can still comment, up/down-vote, or award bounties (via privileges) on the original. These options already exist and can be outlets to express a preferred answer.
I still am not totally satisfied with the "here's existing info, take this and leave" message we may seem to give off from the current duplication function. But combing duplicates out of the site is a priority, and with that comes an implied principle that duplicate questions do not deserve to remain on the site. By that logic, it makes sense that dupe posters don't have input on the existing original question. Again, that's maybe not the friendliest way to handle new posters who were asking about things in good faith, but we have to get rid of duplicates and thus we can't worry too much about giving specific answers to each individual who asks about topics that have already been covered. As a site, I think we're trading off some of the personalized nature of interaction and individually-tailored answers in the name of a tidier database with fewer, higher-quality answers to each question.
Regarding the subjective nature of marking things as duplicates, it can be tricky to tell sometimes whether two questions are the same. Sometimes, they will be about the same topic, but will yield two valuable and different sets of answers. Or one question encompasses the other, but the narrower one gives a more precise answer with unique info where the more general question may be lacking specific details in its answers. Occasionally, two questions may be superficially dissimilar, but the answer to one serves perfectly as the answer to the other. And it's not uncommon for the duplicate poster to disagree with those who voted. Luckily, there is recourse available. When this happens to me as a question asker, I try to do a couple of things:
- Look for existing info on the site about the topic before I write my question, see if I've already been answered.
- If I found some questions discussing a similar subject, I might try to head off dupe votes before they occur by mentioning the earlier question and why I think my question is different enough to remain open.
- If the vote already occurred, I might edit it to make the existing differences more clear (or simply to rewrite it in a way that makes it a unique question, if it turns out they were in fact too similar as written) and explain in the same way I did for the above case. Editing the question will also put it into the Reopen review queue, if I remember correctly, and leaving a comment explaining your stance surely never hurts.
- If there's cause to do so, I might perhaps take my case up on Meta. Duplicate questions can be fairly subjective, so sometimes it's nice to get a more detailed explanation and/or a variety of reasons why your question is a duplicate.
- And sometimes you just gotta give up, too. It very well may be the case that no one here agrees with you or believes that your question is different enough, and sometimes I just try to have some blind faith in the idea that my question being closed by the community is making the site a better place, even if I may not understand how or why!