This question already has an answer here:
Suppose there are two questions that are very obviously duplicates of one another. The one that came first has maybe a few votes and no answers. The second has double the votes of the first and already has two answers. Obviously having two copies of the same question floating around isn't ideal, but what's not so clear is: which one should be closed?
I faced this decision a few minutes ago with this and this. I ended up closing the second, more popular one, but I'm wondering if I made the right choice. The author of the second post wasn't aware of the existence of the first post and unwittingly duplicated it. Granted, the idea time to catch this would have been when the post was first made, but obviously this can't happen all the time. The fault does lie with the person that posted the duplicate, but I'm wondering if this isn't an optimization for personal good at the expense of the good of the community.
I know that I wouldn't appreciate a post I authored to be closed as a duplicate of a post that came later, but at the same time there was already interest and activity around the duplicate, whereas the response to the original was much more limited.
There is an option moderators have to merge posts together. Though I've never tried it (so someone correct me if I'm wrong), I think that it copies all the answers from the duplicate into the master and deletes the duplicate question. In this situation it might be advantageous to move the responses, but I question if it should be used as a general rule.
In any case, I'd like to get some community feedback on this issue. Did I do the right thing? Should I have closed the other one? Or should I just have left both alone?