Seth is partially right here. There are two different reasons that the "exact duplicate" would be shown:
The question was closed as a duplicate a while ago (specifically before about February 2013)
The question that was closed as a duplicate had no upvoted answers.
You can read more about this in animuson's Meta Stack Exchange answer (which I've quoted the relevant parts):
Appears on old questions that were marked as duplicates before the
new closing overhaul. These notices were edited directly into the
question by the Community user and could be freely edited and even
removed. This has since been replaced, but questions that previously
had it were not updated.
This question already has an answer here:
The new default message that is shown to everyone when a question gets
closed as a duplicate. It is an automatic box that is displayed and
cannot be edited.
This question is an exact duplicate of:
Looks like this is a special case for when you close as a duplicate of
another question that doesn't have any upvoted answers.