Some Card packs that you receive will contain duplicate cards. These are cards you have already received before, and they don't count towards the completion of sets. But there are other ways you can make use of them!

• Gather duplicate cards to unlock chests with rewards
  As you accumulate duplicate cards, their star values add up. Collect more card stars to unlock chests with rewards. Duplicates of 5-star Gold cards are extra valuable: they give you 10 stars for unlocking chests.

• Exchange with other players
  You can send duplicate Silver cards to your friends and teammates. Rarer Gold cards, however, can't be sent.