App.xaml.cs

using System;
using System.Collections.Generic;
using System.Configuration;
using System.Data;
using System.Diagnostics;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using System.Windows;

namespace APCUPS
{
    /// <summary>
    /// Interaction logic for App.xaml
    /// </summary>
    public partial class App : Application
    {

        public App()
        {

            string procName = System.Reflection.Assembly.GetExecutingAssembly().GetName().Name;

            // get the list of all processes by the "procName"       
            Process[] processes = Process.GetProcessesByName(procName);

            if (processes.Length > 1)
            {
                Application.Current.Shutdown();
            }
            else
            {
                new MainWindow();
            }
        }
    }
}