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I have seen a few questions which have been marked as duplicate although they the questions have been asked before the question they are marked duplicate of.
Another example -
Isn't it necessary that the time of asking should be considered before marking a question as duplicate.
But also what I can see is that the newer question has better answers than the older ones. But this is unfair to the asker.
How should StackOverflow deal to such problems?
One solution of which I could think of is merging questions. I know that moderators have merging question permission but it's hardly used and according to me, it should be done on every question marked as duplicate.
So instead a marking a question as duplicate and closing it, merge the questions together. Older reputations earned by the question should be retained by the user while newer ones earned after
merging the question should be retained by the person who asked the question earlier. This might probably work on all the questions marked as duplicate, so when a user clicks on link of question which is marked as duplicate, he should be redirected to the original question (message of redirection could be shown). Answers should be merged too.
This will prevent users from earning free reputation by rephrasing a question.