PriorProcess.cs

public static Process PriorProcess()
// Returns a System.Diagnostics.Process pointing to
// a pre-existing process with the same name as the
// current one, if any; or null if the current process
// is unique.
{
    Process curr = Process.GetCurrentProcess();
    Process[] procs = Process.GetProcessesByName(curr.ProcessName);
    foreach (Process p in procs)
    {
        if ((p.Id != curr.Id) &&
            (p.MainModule.FileName == curr.MainModule.FileName))
            return p;
    }
    return null;
}

protected override void PrepareApplication()
{
    var priorProcess = PriorProcess();
    if (priorProcess != null)
    {
        ShowWindow(priorProcess.MainWindowHandle, 1);
        SetWindowPos(priorProcess.MainWindowHandle, new IntPtr(-1), 0, 0, 0, 0, 3);
        SetForegroundWindow(priorProcess.MainWindowHandle);
        Environment.Exit(1);
    }
    else
    {
        base.PrepareApplication();
    }
   
}