Got duplicate in a booster despite already having them

[Arknives-1216]

Were these packs from the Season reward chest? There was a blue post about it:

Duplicate Cards in Season Rewards Chest

Hey everyone, We’ve seen reports of players receiving duplicate cards from their April Rewards Chest. We’ve investigated this and determined that duplicate protection was not applied to individual cards in chest rewards, just packs. For the May Rewards Chest, we will be implementing an improvement that does the duplicate check on the chest level, which will result in all chest rewards being covered by the new protection rules.