I was wanting to know, what traditional mode does the cards come from for Duals? Is it based on which one you have selected?
Just look at the sets it allows when building a deck
Duels allows both wild and standard cards. It has nothing to do with who you select or what cards you have or anything. It’s simply decided by blizz. The specific sets used currently can be found here
https://hearthstone.fandom.com/wiki/Duels#Current_season
Of course, that changes upon set release since (at a minimum) the new set is added to the list. Hopefully they’ll start doing set rotations at some point.