I've read the meta questions on duplicates. This question may well be a duplicate, but there are some unlearned lessons in those answers.
Two questions prompted me to raise this issue in meta.
If the universe is expanding, what is it expanding into?
Apparently this question is used so often as a link target for other questions that a rather large bounty was given to provide better answers. Why?
The first question is a poorly formed question. It asks four distinct questions at once, and the answers are subpar as a result. A google search on "what is the universe expanding into" will yield a number of well-written answers. Our site should be on that list! Each of the four questions raised in that one question should be a question of its own. In fact, some of those questions have been asked individually, and those individually asked questions have received better answers. But yet they are marked as duplicates of the cited poorly written question.
With regard to the second question, the only thing the two questions have in common are that they both ask about Einstein's second postulate of special relativity. The older question asks whether that postulate truly does mean that nothing can go faster than the speed of light. The newer one asks what motivated Einstein to choose that axiom. These are not duplicates. The older question did not ask the latter question, and none of the answers to that older question broach the subject raised in the newer question.
Before marking a question as a duplicate of another one should ask
Is this new question really a duplicate of the target question? Do the answers to the target question answer the question posed in the new question?
If the answer is no, this question is not a duplicate. Don't mark it as such.
Is the target question well posed? Are the answers to the target question well written?
If the answer is no, and particularly if the new question is much better posed than the target, don't mark the new question as a duplicate. Close the old question as trash instead of giving a massive bounty.