Chrome App - Allow only one instance at a time

// background.js
(function() {
    var opened = false;

    chrome.app.runtime.onLaunched.addListener(function() {
        chrome.app.window.create(
            'main.html',
            {
                bounds: { width: 768, height: 700 },
                minWidth: 700,
                minHeight: 550,
                frame: "none"
            },
            function(win) {
                if (opened) {
                    win.close();
                    return;
                }

                win.onClosed.addListener(function() {
                    opened = false;
                });

                opened = true;
                win.show();
            }
        );
    });
})();