The Duplicate Crowns feature is present in the classic game mode and will give you the possibility of obtaining two crowns in exchange for watching an ad.
When you win a character in the classic game, you will see a pop-up window with two options; the first option will say "Continue" and if you press it you will get a crown, while the second option will say "Duplicate" and if you press it, you will get two crowns in exchange for seeing an ad. Remember that you can only use this feature once per turn.
Keep in mind that the feature will not affect the game in any way since you will not double the obtaining of characters, it will only increase the crowns obtained, which are necessary to level up.
For more information on levels, we invite you to read the following article: "What is the use of leveling up?"