This happened few hours ago: the question Series involving gamma function about the sum of a specific series, probably not research level, was followed by another one, Series involving factorials which contains it as a particular case. The new question has been answered, and I believe the previous one becomes obsolete.
I believe one cannot call this a duplicate situation by at least three reasons
the question to be closed preceded the one to be left;
they are not really identical, rather, the one to be closed contains less information;
there are situations when the duplicate should still be left open or merged, while in this case one of the questions is definitely to be closed.
I am sure such situation must be not very rare (although frankly speaking I cannot remember any previous case of it). Now in this particular situation, as I mentioned, most close votes will probably name the "not research level" reason. But for similar cases when the question to be closed is research level, which close reason should be named?
I know I could choose "causes harm" and explain myself. The reason I decided to raise this issue is that I think such situation can be as frequent and typical as duplicate. Does anybody else agree that adding one more close reason would be useful?
In principle one could also modify the "duplicate" close reason to include such situations, but, because of what I said in the last bullet above, I think this is not a good thing to do.