Often when reviewing a question which you simply know it has to be a duplicate, you find the exact same question but it already is marked as a duplicate to a higher quality question with corresponding answer.
I now often flag the question as duplicate of the low quality exact same question asked earlier to give a hint to the OP why it is not a good question. That the OP now has to go to the duplicate of the duplicate to get his answer I do not mind, if he would have looked he would have found it himself and saved us the trouble.
However the purpose of duplicate is to keep it clean. If the question would be closed as a dupe of the dupe, it could end up messy for people searching. When it gets closed it should probably be marked as a duplicate to the high quality question.
So, flag it as duplicate to the exact same question or to the high quality question which might not be so obvious to the OP?