Student or Learner
When Melinda said to the ghost that she is a twin sister of the living girl she just saw, the ghost respondes "Duh". Is it for annoyance or for something natural or what is it for?
Ghost- You can see me, can't you?
mel- Twins.(saying I've seen your twin sister)
"Duh" is an expression used whenever something is so obvious that it does not require saying.
Duh - Definition and More from the Free Merriam-Webster Dictionary
It is not formal English and would only be used among friends or peers. To a stranger it is very rude. (You are basically telling them that you think they just said something stupid.)