Program.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace KeyboardMixer
{
    static class Program
    {

        private static readonly String appGuid = "d2a47dfe-fc58-4b77-8ac7-0c6f14ee2516";

        /// <summary>
        /// The main entry point for the application.
        /// </summary>
        [STAThread]
        static void Main()
        {
            //Mutex prevents multiple instances of the application from running at once
            using (Mutex mutex = new Mutex(false, "Global\\" + appGuid))
            {
                if (!mutex.WaitOne(0, false))
                {
                    MessageBox.Show("Instance already running", "KeyboardMixer");
                    return;
                }

                Application.EnableVisualStyles();
                Application.SetCompatibleTextRenderingDefault(false);
                Application.Run(new MainForm());
            }
        }
    }
}