保存视频时，AlertController会出现两次

Question

这是我的代码：

            let library = ALAssetsLibrary()

            library.writeVideoAtPathToSavedPhotosAlbum(NSURL(string: path), completionBlock: { (url, error) -> Void in
                if error != nil {
                    let error = UIAlertController(title: "Error!", message:
                        "Your video is NOT saved!", preferredStyle: UIAlertControllerStyle.Alert)
                    error.addAction(UIAlertAction(title: "Shit!", style: UIAlertActionStyle.Default,handler:
                        { action in navigationController!.popViewControllerAnimated(true) }))
                    self.presentViewController(error, animated: true, completion: nil)
                } else {
                    let alertController = UIAlertController(title: "Done!", message:
                        "Your video is saved!", preferredStyle: UIAlertControllerStyle.Alert)
                    alertController.addAction(UIAlertAction(title: "Nice!", style: UIAlertActionStyle.Default,handler:
                        { action in navigationController!.popViewControllerAnimated(true) }))
                    self.presentViewController(alertController, animated: true, completion: nil)
                }
            })

如果我按下＆＃34; OK＆＃34;那么说保存视频的AlertController会出现两次。真的很快（如果我等了几秒钟，一切都很好）。问题是什么，我该如何解决？