OAAT for PocketMine-MP.

OAAT.php

<?php
/**
 * @name OAAT
 * @main oaat\OAAT
 * @version 1.0.0
 * @api 1.12.0
 * @load POSTWORLD
 * @author Gamecrafter
 * @description Only one user with this name can join.
 * @link https://gist.github.com/Gamecrafter/f1c0f1b11b97450c776b1f8dff32f690
 */
namespace oaat{
    use pocketmine\event\player\PlayerPreLoginEvent;
    use pocketmine\event\Listener;
    use pocketmine\plugin\PluginBase;
    class OAAT extends PluginBase implements Listener{
        public function onEnable(){
            $this->getServer()->getPluginManager()->registerEvents($this, $this);
        }
        /**
         * @param PlayerPreLoginEvent $event
         * @priority MONITOR
         * @ignoreCancelled true
         */
        public function onPlayerPreLogin(PlayerPreLoginEvent $event){
            $name = $event->getPlayer()->getName();
            if($this->getServer()->getPlayer($name)){
                $event->setCancelled(true);
                $event->setKickMessage("Someone with the name \"$name\" is already online.");
            }
        }
    }
}