Number of simultaneous processes limit

Try placing the following code as the first lines in your Main() method:

using System;
using System.Diagnostics;

static class Program
{
     static void Main(string[] args)
     {
        Process current = Process.GetCurrentProcess();
        Process[] procs = Process.GetProcessesByName(current.ProcessName);
        if (procs.Length > 1)
        {
           // silently kill this instance if another already running
           return;
        }
        // rest of code
     }
}