The only time the game asks you what to do with a duplicate is when it comes via a wheelspin. If it comes as a seasonal event reward or gift it goes straight in your inventory.
So there is a system in place, but why they didn’t extend it to all methods of getting cars, only the devs know.
The problem is exacerbated in that if you don’t wish to dump the cars as gifts you can try to auction them off, but half the time there’s no buyers, even bots and it just becomes a hassle to reclaim and then auction again.