Good vs Evil: Possession

So far, I have many different paths I want to take the idea of being possessed by a doppelgänger or spiritual being.
Idea 1:
It is about a couple, where the woman is possessed by her evil  doppelgänger once a month and torments them through the night. The story ends with the boyfriend getting rid of his girlfriend instead of the doppelgänger.

Idea 2:
A film about what it could look like if a doppelgänger was present in the real world, for example, what sounds or mood will it bring? Would it speak, or just be present? How would the human feel confronted with their doppelgänger  These are the questions that I would like to answer. I feel I could have more fun with this idea because the storyline is simply showing what happens when the doppelgänger appears.

For now, I shall keep on researching and choose an idea and start storyboarding it. I am going to experiment with cloth simulation in video and Maya so I can understand how I want this phantom creature to act.

Below is a story about an occurrence of a doppelgänger in a school.

http://www.historiesmysteriesandstrangeness.com/2013/08/the-doppelganger-of-emilie-sagee.html

http://listverse.com/2013/08/30/10-disturbing-tales-of-doppelgangers/