Code

package me.phantom64.teamdeathmatch.listeners;

import me.phantom64.teamdeathmatch.TeamDeathMatch;
import me.phantom64.teamdeathmatch.utils.GameManager;
import me.phantom64.teamdeathmatch.utils.TeamManager;

import org.bukkit.entity.Arrow;
import org.bukkit.entity.Player;
import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.event.entity.EntityDamageByEntityEvent;

public class EntityDamageByEntity implements Listener {

    private TeamDeathMatch plugin;

    GameManager gm = TeamDeathMatch.getGameManager();
    TeamManager tm = TeamDeathMatch.getTeamManager();

    @EventHandler
    public void onPlayerDamageByPlayer(EntityDamageByEntityEvent e) {
        if (e.getEntity() instanceof Player) {
            Player p = (Player) e.getEntity();
            if (e.getDamager() instanceof Player) {
                Player damager = (Player) e.getDamager();
                if (gm.isPlaying(p) && gm.isPlaying(damager)) {
                    if (tm.getTeam(p) == tm.getTeam(damager)) {
                        e.setCancelled(true);
                        damager.sendMessage("§cThat's your teammate!");
                    }
                }
            }
        }
    }

    @EventHandler
    public void onPlayerDamageByArrow(EntityDamageByEntityEvent e) {
            if (e.getEntity() instanceof Player) {
                Player p = (Player) e.getEntity();
                if (e.getDamager() instanceof Arrow) {
                    Arrow arrow = (Arrow) e.getDamager();
                    if (arrow.getShooter() instanceof Player) {
                        Player shooter = (Player) arrow.getShooter();
                        if (gm.isPlaying(p) && gm.isPlaying(shooter)) {
                            if (tm.getTeam(p) == tm.getTeam(shooter)) {
                                if (shooter == p) {
                                    e.setCancelled(true);
                                } else if (shooter != p) {
                                    e.setCancelled(true);
                                    shooter.sendMessage("§cThat's your teammate!");
                                }
                            }
                        }
                    }
                }
            }
        }

}