// global function:
HWND GetRunningWindow() {
	// Check if exists an application with the same TITLE as this application
	HWND hWnd = FindWindow(NULL, "My TopWindow Title");
	if (IsWindow(hWnd))	{
		HWND hWndPopup = GetLastActivePopup(hWnd);
		if (IsWindow(hWndPopup))
		hWnd = hWndPopup; // Previous instance exists
	} else {
		hWnd = NULL; // Previous instance doesnt exist
	}
	return hWnd;
}

GUI_APP_MAIN
{
	// Checks for previous instance of our app
	HWND hOtherWnd = GetRunningWindow();
	// Allow only one app instance
	if (hOtherWnd) {// hOtherWnd != NULL -> Previous instance exists
		PromptOK("The program is already running");
		SetForegroundWindow(hOtherWnd); // Activate it &
		if (IsIconic(hOtherWnd))
			ShowWindow(hOtherWnd, SW_RESTORE); // restore it
		return; // Abort current app execution
	}
	
// ... otherwise, run app here ...

}