unity打包出的EXE只能单例打开，不能多开。

一、代码实现
using System;
using System.Diagnostics;
using System.Runtime.InteropServices;
using UnityEngine;

/// <summary>
/// 防止同时运行多个实例
/// 用于检查此应用程序是否有多个存在，当检测到另一个实例已在运行，并退出当前的应用程序
/// 仅限于Windows（其他平台由相应的操作系统处理）
/// </summary>
public class PreventMultipleExecution : MonoBehaviour
{
    public static PreventMultipleExecution instance;
   // bool unrun=false;

    [DllImport("User32.dll", SetLastError = true, ThrowOnUnmappableChar = true, CharSet = CharSet.Auto)]
    public static extern int MessageBox(IntPtr handle, String message, String title, int type);

    
    void Awake()
    {
#if !UNITY_EDITOR
        if (PreventMultipleExecution.instance != null)
        {
            Destroy(gameObject);
            return;
        }
        instance = this;
        DontDestroyOnLoad(gameObject);
        string strProcessName = System.Diagnostics.Process.GetCurrentProcess().ProcessName;
        UnityEngine.Debug.Log(strProcessName);
        Process[] app = Process.GetProcessesByName(strProcessName);
        if (app.Length > 1)
        {
            UnityEngine.Application.Quit();
            MessageBox(IntPtr.Zero, "程序正在运行中", "警告", 0);
            return;
        }
       
#endif
    }
}

二、方法二：更简单的方法：直接unity中Player Setting