Hearthstone’s economy works fairly simply - there can be two copies of any card in your deck, or one in the case of Legendary cards. Any spares are useless to you and can be disenchanted for dust, a currency that can then be spent on crafting other cards. There’s even a button to do this for you, getting rid of all your spares in one.
Octavian “Kripparian” Morosan, one of the game’s most popular streamers, has so many spare cards that if he had pressed this button before Blizzard’s server upgrade, it would have brought the whole game down.
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The tidbit comes from Hearthstone game director Ben Brode posting on Reddit, replying to a thread about Kripp’s legendary Collection - which he has been gathering until he has enough cards to create a fully golden collection in one go.