Sorry to seem cynical but this is wholly subjective, and personally I don't think any one car is any more rare than another, fact is that if you are waiting for one very specific model, be that some iteration of the Miata or an R92 LMP, you could be waiting a long time as there are over 1000 standard cars.
That's why you get someone saying "I never saw this car" and someone else saying "I saw that like 6 times"...stands to reason.
The "rarity" value on a certain other site is purely based on how many own that car, so is more of a popularity contest, and is really misleading.
I do find it odd however how duplicates of the same car come up at the same time sometimes, but I guess this is how the cycling of the car works, like it chooses "on this day there's going to be a choice from cars x, y, z etc" and this can include dupes.
I got two R8 race cars in new colours just today, expensive but very nice!
