static void Main()
  {
     string mutex_id = "MY_APP";
     using (Mutex mutex = new Mutex(false, mutex_id))
     {
        if (!mutex.WaitOne(0, false))
        {
           MessageBox.Show("Instance Already Running!", "Error", MessageBoxButtons.OK, MessageBoxIcon.Hand);
           return;
        }
        // Do stuff
     }
  }

private static bool AlreadyRunning()
{
    Process[] processes = Process.GetProcesses();
    Process currentProc = Process.GetCurrentProcess();

    foreach (Process process in processes)
    {
        try
        {
            if (process.Modules[0].FileName == System.Reflection.Assembly.GetExecutingAssembly().Location
                        && currentProc.Id != process.Id)
                return true;
        }
        catch (Exception)
        {

        }
    }

    return false;
}

private static bool AlreadyRunning()
    {
        Process[] processes = Process.GetProcesses();
        Process currentProc = Process.GetCurrentProcess();
        logger.LogDebug("Current proccess: {0}", currentProc.ProcessName);
        foreach (Process process in processes)
        {
            if (currentProc.ProcessName == process.ProcessName && currentProc.Id != process.Id)
            {
                logger.LogInformation("Another instance of this process is already running: {pid}", process.Id);
                return true;
            }
        }
        return false;
    }

bool createdNew;
using (Mutex mutex = new Mutex(true, "MyApplicationName", out createdNew))
{
    if (createdNew) {
         // process
    }
    else {
         // in my case, quietly exit
    }
}