@EventHandler
public void onPlayerDamage(EntityDamageByEntityEvent e) {
Entity damager = e.getDamager();
Entity damageTaker = e.getEntity();

if (damageTaker instanceof Player) {
    //DamageTaker is a Player
    Player taker = (Player) damageTaker;
    if (damager instanceof Player) {
        //Damage Causer is also a player
        Player damagerPlayer = (Player) damager;
    }
}