I've posted an answer for this Question, Today I found the same question with no answer here (this question is too old). So can I post my answer here too? or is it fine to post the link to the question which I have answered? If I do so, one question seems to be duplicate. So how to handle this?
No, if you are sure it is basically the same question, flag it as duplicate of the answered one, even if it is older. We assume that answered question was somehow better (at least better to attract answer) so don't bother with age, but flag the unanswered one.
Now if questions are substantially different and by accident the same answer applies to both of them, it is OK to post an answer to each question. Just don't copy it verbatim: Tailor it for the question you are answering.