>This can be solved via two strategies. The first, described in broad terms under the name “Slasher” here and developed further by Iddo Bentov here, involves penalizing validators if they simultaneously create blocks on multiple chains, by means of including proof of misbehavior (ie. two conflicting signed block headers) into the blockchain as a later point in time at which point the malfeasant validator’s deposit is deducted appropriately. [...] The second strategy is to simply punish validators for creating blocks on the wrong chain.

Which one does ethereum use?