You must have at least two deck recipe to achieve this glitch, naming recipe A and recipe B

1) Go to the deck/bag screen

2)Load recipe B (touch the menu button above the "OK" button, then the first button seen)

3)Add the card you want to duplicate to deck (max. 49 cards)

4)Take out card(s) (apart from what you just added) from the deck so that the number of cards is 50

5) Register the new formed deck recipe to deck recipe A (touch the menu button and then the second button seen).

6) You will be asked to overwrite the existed recipe or not, choose yes.

7) Load deck recipe B

You will then find the card you just added increases by the number you added.

BE EXTREMELY CAREFUL not to register the new formed deck recipe to deck recipe B, or you will lose card instead of gain card.