You can accumulate Duplicate cards to trade them in for chests with rewards.

• Each reward chest in Collection has a value expressed in duplicate card stars. As soon as you accumulate enough card stars from duplicates, the chest becomes available for unlocking.
• If you choose to open the reward chest, you need spend the amount of stars it costs.
• At the same time, duplicate cards themselves also get spent, and disappear from your set.
• Duplicates of Gold cards give you 10 card stars for unlocking chests.

Instead of buying reward chests, you can also keep your duplicate cards to later exchange them with your in-game friends and teammates.