[Seth][1] is partially right here. There are two different reasons that the "exact duplicate" would be shown:

1. The question was closed as a duplicate a while ago (specifically [before about February 2013][2])

2. 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][3] (which I've quoted the relevant parts):

> > **Possible Duplicate:**  
> > [list]
> 
> 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:**  
> > [list]
> 
> 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:**  
> > [list]
> 
> Looks like this is a special case for when you close as a duplicate of
> another question that doesn't have any upvoted answers.


  

[1]: http://meta.askubuntu.com/questions/15470/exact-dupe-vs-dupe#comment28527_15470
  [2]: https://meta.stackexchange.com/questions/166707/changes-to-close-as-duplicate-part-deux
  [3]: https://meta.stackexchange.com/questions/197452/what-decides-what-to-prepend-the-duplicate-question-list-with-on-a-duplicate-que/197454#197454