The Duplicate Crowns feature is found in Classic mode and will give you the opportunity to obtain two crowns in exchange for viewing an advertisement.
When you win a character in the Classic game, you will see a pop-up with two options; the first one will say Continue and if you press it you will get one crown, while the second one will say Duplicate and if you select it, you will be able to get two crowns in exchange for watching an advertisement.
Keep in mind that this feature will not affect the outcome of the game in any way as it will not duplicate the character won, but only increase the crowns gained, which you will need to level up.
For more information on leveling up, we invite you to read the following article: What is the point of leveling up?