List Picker Selection Change - windows-phone-7

Question

I have 2 list pickers which showing item name and batch number. Batch number depends on Item Name. So when selecting Item Name I want to bind Batch numbers to the second list picker.My code like this. I'm writen code for Item list picker 'SelectionChanged' event. This is not working.
private void lstItem_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
if (lstItem.SelectedIndex != 0)
{
int itemId = invoice.listItems[lstItem.SelectedIndex].ItemMasterID;
getBatches = invoice.bindBatchesForItem(itemId);
lstBatch.ItemsSource = getBatches;
}
}
bindBatchesForItem will return the list of batch numbers for selected item.This method returning the batch properly. But 'lstItem_SelectionChanged' event calling twise automatically. First time executing properly and when second time executing attached error comming.
public ObservableCollection<BatchNumbers> bindBatchesForItem(int selectedItem)
{
listBatches.Clear();
var batchList = from DailyItemStock DI in APPCommon.SFADB
where Convert.ToDateTime(DI.StockDate).Date == Convert.ToDateTime(APPCommon.TransactionDate).Date && DI.ItemMasterID == selectedItem
select new
{
DI.BatchNo
};
listBatches.Add(new BatchNumbers() { BatchNo = "0" });
foreach (var lists in batchList)
{
listBatches.Add(new BatchNumbers()
{
BatchNo = lists.BatchNo.ToString()
});
}
batchList = null;
return listBatches;
}