nightmare mode kills you sometimes even if you've made no mistake
It might have been something teeny tiny you missed, like how the nose is drawn. I came across a doppelganger that I knew was a fake cause I had already let the real Barbatos in. I could not figure out what the tell was, other than maybe the nose was off. It was the same shape and size, the only difference was that there was no gap between 2 of the drawn lines. I wouldn't have noticed it except for I had spent 5 minutes comparing everything. I let him in so that the game would tell me what was off, and it was the nose.