Nightmare Frankendoll is not just another doppelganger. He's the core of your nightmares. He also predicted that Frankendoll will come out of nowhere and murder everything. And look at that. It happened.
Nightmare Frankendoll usually appears in your dreams, just to turn it into a nightmare.
Dreaming about money? He came to ruin it.
Dreaming about video games? He's also there to ruin it.
Already having a nightmare? What are the odds. He's going to make it worse.
He is also a hallucination. He can appear anywhere. Your friends might not see him, but you can. And how are you going to explain to everyone that a giant white ghostly version of Frankendoll has been terrorizing you all day? Embarrassing, right?