backgrounddownloader 完成状态的如何检测才没有问题？？？？？？？？？？？

Question

我的程序里面在后台任务中下载了PDF，需要在下载完成时对SQLITE数据库做一些操作，在下载某些大一点的PDF时，完成后，发现被检查了两次，导致插入数据库报约束异常，请问怎么解决？

代码如下：

  private async Task HandleDownloadAsync(DownloadOperation download, bool isNew)
        {
            cgtitle cg = new cgtitle();
            var pdffile = download.ResultFile;
            var cginfo = await cg.GetInfo(pdffile.Name.ToString());
            try
            {
                // 将 DownloadOperation 附加到 TransferModel，以便下载进度可通知
                TransferModel transfer = new TransferModel();
                transfer.DownloadOperation = download;
                transfer.Source = download.RequestedUri.ToString();
                transfer.Destination = download.ResultFile.Path;
                transfer.Progress = "0 / 0";
                transfer.cg_title = cginfo.c_cg_title;
                _transfers.Add(transfer);

                // 当下载进度发生变化时的回调函数
                Progress<DownloadOperation> progressCallback = new Progress<DownloadOperation>(DownloadProgress);

                if (isNew)
                    await download.StartAsync().AsTask(App._cancelToken.Token, progressCallback); // 新增一个后台下载任务
                else
                    await download.AttachAsync().AsTask(App._cancelToken.Token, progressCallback); // 监视已存在的后台下载任务

                if (download.Progress.Status == BackgroundTransferStatus.Completed)
                {

//这里有时会被重复执行，如果不这样检查，那还有其它的检查方法有呢？？？？？？？？？？？？？？？？？？？？？？？？？

                    Debug.WriteLine(cginfo.i_generic_cg);

                     var pdffolder = await Utils.GetPdfFolder();
                    await pdffile.CopyAsync(pdffolder, pdffile.Name, NameCollisionOption.ReplaceExisting);
                    if (Common.Utils.IsNetworkAvailable())
                    {
                        try
                        {
                            await updateCgPdf(cginfo.i_generic_cg);
                        }
                        catch { }
                    }
                    showtip(cginfo.c_cg_title, "下载完成");
                }
                else
                {
                    showtip(cginfo.c_cg_title, "下载失败");
                }
            }
            catch (Exception ex)
            {
                // 将异常转换为 WebErrorStatus 枚举，如果获取到的是 WebErrorStatus.Unknown 则说明此次异常不是涉及 web 的异常
                WebErrorStatus error = BackgroundTransferError.GetStatus(ex.HResult);
                showtip(cginfo.c_cg_title, "下载失败");
            }
            finally
            {
                _transfers.Remove(_transfers.First(p => p.DownloadOperation == download));
            }
        }