Because of a [recent question][1] I've been wondering what duplicate actually means.

Researching the issue here and on meta.SE has not given me a conclusive answer [but I did notice my own thoughts reflected a few times.][2]   
I've also noticed some cases of people [disagreeing with what a duplicate is.][3]

So I'm wondering if we can put a definite answer to:

 1. Are duplicates duplicate questions
 2. Are duplicates duplicate answers
 3. When do we close as duplicate  

I'm of the idea that there probably isn't some line in the sand that determines a duplicate. But there should be a clear idea of when something is a duplicate, and when something is a unique and connected issue.


  [1]: https://meta.serverfault.com/q/8288/218888
  [2]: http://meta.stackexchange.com/a/74090/276684
  [3]: https://meta.serverfault.com/questions/5868/explanation-of-duplicate