如何修改iOS中的图像元数据(EXIF)而不重复？

if( [self.textView.text length] != 0 && ![self.userComments isEqualToString: self.textView.text])
        {
            // This code works but creates a duplicate image
            NSMutableDictionary *userCommentDictionary = [NSMutableDictionary Dictionary];
            [userCommentDictionary SETVALue:self.textView.text forKey:(NSString *)kCGImagePropertyExifUserComment];

            NSMutableDictionary *Dict = [NSMutableDictionary Dictionary];
            [Dict SETVALue:userCommentDictionary forKey:(NSString *)kCGImagePropertyExifDictionary];

            ALAssetsLibrary *al = [[ALAssetsLibrary alloc] init];

            [al writeImageToSavedPhotosAlbum:[self.imageView.image CGImage]
                                Metadata:Dict
                         completionBlock:^(NSURL *assetURL,NSError *error) {
                             if (error == nil) {
                                 NSLog(@"Image saved.");
                                 self.userComments = self.textView.text;
                             } else {
                                 NSLog(@"Error saving image.");
                             }
                         }];
        }

反正有没有避免重复？
谢谢你的时间