Once purchased, the Double Winnings bonus instantly doubles all prize money throughout the game. An easy way to verify if it is working is to check the listed prize money for the first race of the beginner cup (tarmac). If it's listed at $200, the bonus is working. Note: Unfortunately on the race results screen it won't show the doubled pickups, however if you add it all up after you win a race the double is still applied to the pickups!
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