You’re the Head Judge of a Standard PPTQ. While verifying that you have all the lists, you notice that Percy’s list has “2x Ajani” on it. You go to Percy’s match and notice that he has one Ajani Unyielding in his graveyard, and another in his hand. What do you do?
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Because this decklist error was not discovered during a deck check, the penalty should not be applied until the beginning of the next round. Wait until Percy’s match is finished, then explain the penalty, and update his decklist to match his deck, being sure to check for any other errors as well. The penalty will be enforced at the start of Percy’s next match.