Earlier quoted context omitted.
> How do you intend to prevent that? Very very few posts are actually uneditable at all, and editing is open to all users (some need approval). If a post is locked and outdated it's likely that the question was locked because it was the kind of question that does not provide long-lasting answers (for example a software recommendation list).
Given that a voting system is in place, why not let the duplicate stay open and just mark it as a duplicate and link it to the other thread? People who don't like the new thread can down vote it and people who do want to contribute meaningful replies can still do and be upvoted if it adds value. I don't see how this might affect SEO in any bad way than the current scheme does.
If duos were open, the you'd have to look in all the places first to find the best answer. And then you might as well skip that step and ask another question rather than search.
Try searching a forum or /r/javahelp or the like for an answer. See how many results you have to hit before you find an answer that works for you. Then reconsider the value of closing and pointing people at the duplicate question.