When you receive a card you already own, it becomes a duplicate. The number of duplicates you have is shown as a counter on the card.
Duplicates convert into duplicate stars based on the card’s rarity (gold cards give double stars).
You can spend duplicate stars to buy Duplicate Chests.
You can send duplicate cards to your friends, except for gold cards which cannot be sent.
You can send up to 3 duplicate cards per day. If you reach the limit, a cooldown timer will show until midnight server time.
If you don’t have duplicates of a card, the send button will be disabled with a message explaining you need a duplicate to send.
You can only send duplicates to friends who have the Collection feature unlocked.
When you receive duplicates from friends, your duplicate stars balance increases accordingly.