func applicationDidFinishLaunching(aNotification: NSNotification) {
    /* Check if another instance of this app is running. */
    let bundleID = Bundle.main.bundleIdentifier!
    if NSRunningApplication.runningApplications(withBundleIdentifier: bundleID).count > 1 {
         /* Show alert. */
         let alert = NSAlert()
         alert.addButton(withTitle: "OK")
         let appName = Bundle.main.object(forInfoDictionaryKey: kCFBundleNameKey as String) as! String
         alert.messageText = "Another copy of \(appName) is already running."
         alert.informativeText = "This copy will now quit."
         alert.alertStyle = NSAlert.Style.critical
         alert.runModal()

         /* Activate the other instance and terminate this instance. */
         let apps = NSRunningApplication.runningApplications(withBundleIdentifier: bundleID)
             for app in apps {
                  if app != NSRunningApplication.current {
                      app.activate(options: [.activateAllWindows, .activateIgnoringOtherApps])
                      break
                  }
             }
                NSApp.terminate(nil)
         }   
       /* ... */
}