Kaervek, the Punisher double copies
Was playing against opponent who had a single creature on the board which was this new Kaervek, the Punisher from OTJ.
Text says: Whenever you commit a crime, exile up to one target black card from your graveyard and copy it. You may cast the copy. If you do, you lose 2 life.
Opponent managed to copy 2 cast downs that were on the grave after committing a crime, no other permanents on the board that could duplicate the effect
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Rezzahan#77802 commented
Casting Cast Down on one of your creatures is a new crime and triggers Kaervek again. In essence, as long as your opponent has mana and life to pay, as well as black cards in his graveyard that can target you, your creatures, or cards in your graveyard, he can chain them into each other, as each cast is a new crime.