修改：Program.cs

[STAThread]
static void Main()
{Mutex mutex = new Mutex(true, "NewGuid123456", out bool isCreatedNew);if (!isCreatedNew){MessageBox.Show(Application.ProductName + "is running...");return;}Application.EnableVisualStyles();Application.SetCompatibleTextRenderingDefault(false);Application.Run(new Form1());
}

参考：C# 禁止程序重复启动