Hmm, duralumin hasn't really been discovered off of Scadrial I suppose. I figure though, that if it had the same properties as aluminium for the purpose of being Pushed or Pulled, then they would use it instead of aluminum, since copper is cheaper then aluminium. But then, it is possible that the cost to melt the 2 together is higher then the cost of the aluminum. I suppose it would be mildly weird for 4% copper to completely reverse the effects of the metal, but that is what it does when burned in Allomancy. It seems like all of the Allomantic metals have power outside of the metallic arts though. I figure here are some possibilities for it's mechanics.
They have forgotten how to make it on Scadrial. Unlikely I would say, as I'm not sure why they would lose the knowledge, and Connectors are still a thing.
It acts like any other metal when Pushed or Pulled. Possible, would explain not using it for bullets or anything
It acts like aluminum and can't be Pushed or Pulled. Again possible, the 4% copper might not make enough of a difference to make forging it worth it
It is easier to Push or Pull then other metals. Unlikely, as it would be used by Coinshots to shoot even faster bullets I would imagine.