Many items are given instead of one

One item must be awarded after completing the achievement. Depending on the achievement - they are given (1-10)+ items.

Code:

package plugin.grenka.plugin;

import org.bukkit.Bukkit;
import org.bukkit.ChatColor;
import org.bukkit.GameMode;
import org.bukkit.Material;

import org.bukkit.entity.Player;
import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.event.block.Action;
import org.bukkit.event.player.*;
import org.bukkit.inventory.ItemStack;

import org.bukkit.inventory.meta.ItemMeta;
import org.bukkit.plugin.java.JavaPlugin;


import java.util.ArrayList;
import java.util.List;
import java.util.Timer;


public final class Plugin extends JavaPlugin implements Listener {
    boolean ibb = true;



    @Override
    public void onEnable() {
        // Plugin startup logic
        System.out.println("The plugin is starting up.");
        getServer().getPluginManager().registerEvents(this, this);


    }
    @EventHandler
    public void onPlayer(PlayerInteractEvent event) throws InterruptedException {
        Player player = event.getPlayer();
        Action action = event.getAction();
        Timer timer = new Timer();
        if (action.equals(Action.RIGHT_CLICK_AIR) || action.equals(Action.RIGHT_CLICK_BLOCK)) {
            if (player.getItemInHand().getType() == Material.TRIDENT) {
                player.getInventory().getItemInMainHand().setAmount(player.getInventory().getItemInMainHand().getAmount() - 1);
                player.updateInventory();
                player.setGameMode(GameMode.CREATIVE);
                Bukkit.getScheduler().runTaskLater(this, () -> {
                    player.setGameMode(GameMode.SURVIVAL);
                }, 40L);



            }
        }
    }


    @EventHandler
    public void PlayerAdvancementDone(PlayerAdvancementDoneEvent event){
        Player player = event.getPlayer();
        ItemStack item = new ItemStack(Material.TRIDENT);
        ItemMeta im = item.getItemMeta();

        im.setDisplayName(ChatColor.translateAlternateColorCodes('&', "&a&lCreative"));
        List<String> lore = new ArrayList<String>();
        lore.add("Bla-bla-bla");
        im.setLore(lore);
        item.setItemMeta(im);

        player.getInventory().addItem(item);
        player.sendMessage(ChatColor.GREEN + "Give.");
    }
}

 

Looks like that event gets called multiple times.
You can solve this by having a collection of player ids and adding the player to it when the event fires and give them the diamond, but only if their id is not on the list already.

 

Also

Code:

System.out.println("The plugin is starting up.");

this is unnecessary log clutter. The server already puts plugin initialization messaging in the log. You really should also be using the logger, not println.