Almost all cards can be crafted in Collection under Crafting menu.
Duplicates and cards a player does not need can be sold/shattered for two different currencies Stardust (Items and Abilities) and Mistdrops (Creatures , Realms , Shardstones). These currencies can then be used to purchase/craft the cards the player needs.
Stardust and Mistdrops also can be gained as a reward at the end of a Season.