Buying cars in the auction is fun, but buying the same car twice isn’t. As the auction house doesn’t tell you whether or not you already own a car I wrote a quick web app to use on my phone when I’m bidding on things.
It’s basic, and a bit of a work-in-progress, but if you’d find that helpful you can use it too - it lives at https://fh4.ooer.com/
(And if you’re interested in code, the source is available at GitHub - onion2k/fh4-car-tracker: A simple car tracker app for Forza Horizon 4. )