What asked in comments should be something relative to the post: For example, if the user who wrote the post was not clear or wrote something wrong, that should be said in comments. In the latter case, it could be a new answer, as far as the user who writes it doesn't limit the answer to "What wrote by @username is wrong." but it also adds what the correct solution is.
If somebody has a similar question, but different requirements, that should be a different question. The important is that to make in what the new question is different from the old question: If the requirements are different, that should be clearly said; if the user tried what suggested in the other question, without results, that should be said.
Doing so doesn't warrant the new question is closed as duplicate; it could be that even with the new requirements the answer doesn't change. I would make an exception to this: If the question is about a module to use, I would rather leave the question open, even if for both the questions the module to use is the same. That is, if the module that can be used in both the cases is not a module like the Views module, and if the OP doesn't expose compelling reasons not to use that module.
Getting an answer depends all from how the question is written: If it shows research, users could find challenging to answer it; if it is too generic, and the OP doesn't show any previous research, users will probably not find interesting to answer it.
It is not a matter of asking a question similar to what asked from another user; it is a matter of making the question interesting, and not sounding like a "do my job for me" question.