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Today I saw that a question was marked as a duplicate, but the original was also a duplicate. The question marked as a duplicate can be found here. The question that is a duplicate, but never got marked as one can be found here. The real original question can be found here.
So why did the first question get marked as a duplicate, but the second one didn't?
Well, because nobody flagged the question as a duplicate, or cast a close vote. You can see this in the timeline of the post - both actions would have put this in the Close Vote review queue, of which there would be a trace in the timeline. Note that the Close Vote review queue is by far the largest queue, so it might not even be reviewed completely before the votes age away.
Questions will only be closed as a duplicate if:
1) Somebody notices that it is a duplicate
2) Bothers to flag it as such
3) The flag is seen and acted upon before it ages away
It is often the first point - the noticing - that doesn't happen.
Now you've noticed a duplicate, you've achieved the first step. Flag it.
There are probably hundreds of duplicate questions across the SE sites.
