5th Card reward
I'm sick of losing rewards due to earning copies of cards I already have 4 of. When getting a copy of cards I already have 4 of, I think it would be better if I got a wild card of the same rarity instead of a duplicate I can't use. That way I get a reward I earned fair and square and not feel like I lost something due to excessive duplicates.

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[Deleted User] commented
Excess commons and uncommons contribute to the vault. 0.1% for a common, 0.3% for an uncommon. A completed vault (100%) gives you 3 uncommon wildcards, 2 rare wildcards, and 1 mythic wildcard. Depending on how excessively you play the game, or draft, you can complete 1-2 vaults per set.
Excess rares give you 20 gems, excess mythics 40 gems. Though when opening a pack, those rarities are subject to duplicate protection, meaning, while you have not yet completed all rares and mythics you can get from opening packs, you will get a rare or mythic you are still missing. Note, that some cards you cannot get from packs, but have to craft with wildcards instead. Sometimes you can get those from the mastery pass, too. A rare or mythic ICR (individual card reward) is not subject to duplicate protection.