The starks have a plot card that kills all characters. If you have a "duplicate" does this save the character?

thankyou

A duplicate doesn't (by itself) save the character - it gives the controller of the character the ability to, as a response, discard the duplicate to save the character. That means you're never forced to use it - you can choose to let the character die.

In the case of Valar, there's nothing preventing you from using dupes to save character - contrary, to, say, Wildfire Assault (which specifically stipulates that the characters cannot be saved).

Note also that while that plot is part of the Stark deck in the Core Set, it is not a "Stark plot." Generally, you can include any plot in any deck you build. Like events, if a plot is limited to one House or another, it will specifically say, "House (whatever) only."

There are other ways to save as well -- Bodyguard, Ghost, Viserys, Nymeria, Gilded Plate, and others.

The Power of Blood plot prevents characters with the Noble crest from being killed, even by Valar.