Plugin already initialized

I am doing plugin for teleport between servers in bungeecord. Suddenly, i don't know how, but error starts occur. Can you tell me, where is the problem? Thank you for your response.

Full Stacktrace:

org.bukkit.plugin.InvalidPluginException: java.lang.IllegalArgumentException: Plugin already initialized!
at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:139) ~[spigot-1.15.2.jar:git-Spigot-800b93f-8160e29]
at org.bukkit.plugin.SimplePluginManager.loadPlugin(SimplePluginManager.java:334) ~[spigot-1.15.2.jar:git-Spigot-800b93f-8160e29]
at org.bukkit.plugin.SimplePluginManager.loadPlugins(SimplePluginManager.java:253) [spigot-1.15.2.jar:git-Spigot-800b93f-8160e29]
at org.bukkit.craftbukkit.v1_15_R1.CraftServer.loadPlugins(CraftServer.java:351) [spigot-1.15.2.jar:git-Spigot-800b93f-8160e29]
at net.minecraft.server.v1_15_R1.DedicatedServer.init(DedicatedServer.java:203) [spigot-1.15.2.jar:git-Spigot-800b93f-8160e29]
at net.minecraft.server.v1_15_R1.MinecraftServer.run(MinecraftServer.java:784) [spigot-1.15.2.jar:git-Spigot-800b93f-8160e29]
at java.lang.Thread.run(Thread.java:830) [?:?]
Caused by: java.lang.IllegalArgumentException: Plugin already initialized!
at org.bukkit.plugin.java.PluginClassLoader.initialize(PluginClassLoader.java:178) ~[spigot-1.15.2.jar:git-Spigot-800b93f-8160e29]
at org.bukkit.plugin.java.JavaPlugin.<init>(JavaPlugin.java:52) ~[spigot-1.15.2.jar:git-Spigot-800b93f-8160e29]
at pl.piotrek.main.Main.<init>(Main.java:9) ~[?:?]
at jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[?:?]
at jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[?:?]
at jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[?:?]
at java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:500) ~[?:?]
at java.lang.reflect.ReflectAccess.newInstance(ReflectAccess.java:166) ~[?:?]
at jdk.internal.reflect.ReflectionFactory.newInstance(ReflectionFactory.java:404) ~[?:?]
at java.lang.Class.newInstance(Class.java:591) ~[?:?]
at org.bukkit.plugin.java.PluginClassLoader.<init>(PluginClassLoader.java:72) ~[spigot-1.15.2.jar:git-Spigot-800b93f-8160e29]
at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:135) ~[spigot-1.15.2.jar:git-Spigot-800b93f-8160e29]
... 6 more
Caused by: java.lang.IllegalStateException: Initial initialization
at org.bukkit.plugin.java.PluginClassLoader.initialize(PluginClassLoader.java:181) ~[spigot-1.15.2.jar:git-Spigot-800b93f-8160e29]
at org.bukkit.plugin.java.JavaPlugin.<init>(JavaPlugin.java:52) ~[spigot-1.15.2.jar:git-Spigot-800b93f-8160e29]
at pl.piotrek.main.Main.<init>(Main.java:9) ~[?:?]
at jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[?:?]
at jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[?:?]
at jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[?:?]
at java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:500) ~[?:?]
at java.lang.reflect.ReflectAccess.newInstance(ReflectAccess.java:166) ~[?:?]
at jdk.internal.reflect.ReflectionFactory.newInstance(ReflectionFactory.java:404) ~[?:?]
at java.lang.Class.newInstance(Class.java:591) ~[?:?]
at org.bukkit.plugin.java.PluginClassLoader.<init>(PluginClassLoader.java:72) ~[spigot-1.15.2.jar:git-Spigot-800b93f-8160e29]
at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:135) ~[spigot-1.15.2.jar:git-Spigot-800b93f-8160e29]
... 6 more

Main class:

package pl.piotrek.main;
import org.bukkit.plugin.java.JavaPlugin;
import pl.piotrek.commands.ServersCommand;
import pl.piotrek.events.ServersGuiListener;
import pl.piotrek.gui.Gui;
import pl.piotrek.message.channel.PluginMessagingListener;
import pl.piotrek.message.command.ConnectCommand;
public class Main extends JavaPlugin {
private static Main instance;
public static Main getInstance() { return instance; }
public void setInstance(Main instance) { Main.instance = instance; }
public Gui gui;
@Override
public void onEnable() {
System.out.println("Enabled TeleportationUtils v1.0.0");
setInstance(this);
registerCommands();
registerListener();
this.getServer().getMessenger().registerOutgoingPluginChannel(this, "BungeeCord");
this.getServer().getMessenger().registerIncomingPluginChannel(this, "BungeeCord", new PluginMessagingListener());
gui = new Gui();
}
public void registerCommands() {
getCommand("connect").setExecutor(new ConnectCommand());
getCommand("servers").setExecutor(new ServersCommand());
}
public void registerListener() {
this.getServer().getPluginManager().registerEvents(new ServersGuiListener(), this);
}
}

Plugin.yml:

name: TeleportationUtils
version: 1.0.0
main: pl.piotrek.main.Main
api-version: 1.15
commands:
connect:
aliases: [cnt]
description: Connect to another sevrer command
servers:
aliases: [svr]
description: Server menu command for teleport between servers
permissions:
commands:
connect:

Your stack trace shows that your plugin is initially initialized at pl.piotrek.main.Main.<init>(Main.java:9) and then it's trying to be initialized again at the same point. It is very likely that you have the same plugin twice in your plugins folder, probably under a different name.

Have a look through your server's plugin folder and find the duplicate.

you most likely have "extends JavaPlugin" in 2 classes. you can only have this in your Main class. What you should do instead is

    private final Main plugin;
public className(Main plugin) {
this.plugin = plugin;
}

in your other classes, and use plugin.whatever to access things you normally only could in your Main class.

Also in your plugin.yml, you have

permissions:
commands:
connect:

this does not follow Bukkit's permission standards which can be found here.

And when naming your packages, its best to put the plugin name in there. something like pl.piotrek.TeleportationUtils.main.Main so if you have more then one of your plugins in the server, it won't cause issues.