NET不能同时打开同样的两个程序

static void Main()
{
Process instance = RunningInstance();
if (instance == null)
  {
   Application.Run(new Form1());
  }

}

#region RunningInstance
private static Process RunningInstance() 
{
  Process current = Process.GetCurrentProcess();
  Process[] processes = Process.GetProcessesByName   (current.ProcessName);
  if(processes.Length>1)
  {
   return current;
  }
  else
  {
   return null;
   }
}
#endregion