A couple thoughts:
First, there are several low cost cars that I have never won.
I have 34 cars under $100K that have zero dupes (I would say about 20 of these were purchased.) This is quite substantial, since the vast majority of the cars are $300K - $1M range.
Meanwhile, I have received several duplicates of substantially more expensive cars.
The really egregious ones are in the $500K range. 8 GT-Rs Schulze, 7 focus gr.b rally, 8 atenza gr.4, etc.
Yet, I can't win the Honda Beat, or the Fit, the Eunos Roadster, Fiat 500F, Integra Type R, Trueno, Mini Cooper S, etc.
I'm assuming the probability is not completely based on value/price.