@Background Pony #1773  
Totally agreed.  
It makes Luna a much more interesting character when she’s responsible for everything about Nightmare Moon.  

In the show, she’s had plenty of opportunities to make it clear that Nightmare Moon was a think that happened to her and not a thing she did were that truly the case. She always puts it in the “thing she did” category; we’ve got it straight from the horse’s mouth. [shot]) In show continuity, Nightmare Moon is Luna’s greatest mistake, not an outside “force.” Comics and show are a bit different, that’s nothing new. Nightmare Rarity is pretty awesome, though.

Nightmare Moon almost seems surprised to be called Luna.
 
I don’t think this is evidence enough that they are separate individuals, but that also depends on what you think of the nightmare force in the comics (though one still has to agree to give in with those things).