If i have a stark character attacch as a dupe of jory casel and my opponent throw a nightmare to him ,, He loose his dupe?

I don't see why. Once a card becomes a dupe, it loses all text, titles, and traits. Once the dupe was legally attached, it would be a dupe. Blanking him would have no effect.

Just seconding what Doulos said.

Jory's ability ends once the duplicate is attached and the duplicate is now just a duplicate. Blanking him does nothing to the duplicate itself (meaning it doesn't prevent it from being used to save him) as it is literally just a duplicate now.