grid 绑定会重复叠加数据

     <ScrollViewer Height="633" HorizontalAlignment="Left"  Name="scrollViewer1" VerticalAlignment="Top" Width="441">
                    <!--show score-->
                    <Canvas x:Name="CanvasScores">
                        <TextBlock Text="{Binding Path=LocalizedResources.productName, Source={StaticResource LocalizedStrings}}" Canvas.Top="50" Canvas.Left="0" />
                        <TextBlock Text="{Binding Path=LocalizedResources.barCode, Source={StaticResource LocalizedStrings}}" Canvas.Top="50" Canvas.Left="200" />

                        <TextBlock x:Name="TextBlockID" Canvas.Top="100" Canvas.Left="0" />
                        <TextBlock x:Name="TextBlockName" Canvas.Top="100" Canvas.Left="200" />
                       

                    </Canvas>

                </ScrollViewer>

            </Grid>

 protected override void OnNavigatedTo(NavigationEventArgs e)
        {
           
            loadData();
        }
        private void loadData()
        {
           if(belong==null||belong.Length==0)
           {
               belong = "product";
           }
            string strSelect = "SELECT * FROM aa";
            if (_invoiceEntries!=null)
            {
                _invoiceEntries.Clear();//为什么clear 不了
            }
            _invoiceEntries = (Application.Current as App).db.SelectObservableCollection<productType>(strSelect);
            int count = _invoiceEntries.Count;
            foreach (productType data in _invoiceEntries)
            {
                TextBlockID.Text += data.Id + Environment.NewLine;
               // string Typename = (Application.Current as App).db.GetSingle("select typename from producttype where id=" + data.Id);
                TextBlockName.Text += data.typeName + Environment.NewLine;
               
            }
                 //假如之前已经有10行，再添加一行返回该页面时就会绑定21行。。。有10行重复了


        }

[解决办法]
返回页面时navigatedto也执行了，这个这么简单的问题，跟踪一下就知道了，你要判断navigatedto是第一次进入，还是从其他页面导航进入，还是从墓碑恢复来的。。。