The Duplicate Crowns feature can be found in the classic game mode and it allows you to get two crowns by watching an ad.
When you obtain a character in the classic game, a pop up window with two options will be displayed. The first option will be a 'Continue' button that, when tapping on it, will give you a crown, and the second one will be a 'Duplicate' button that, when tapping on it, will give you two crowns if you watch an ad. Remember that you'll only be able to use this feature once per turn.
Bear in mind that the feature won't affect the game in any way because you won't duplicate the characters you obtain, but the crowns, which you'll need to level up.