adding unique items in listview

Hi,

You probably have some sort of ID that uniquely identifies each product. You could store it in the Tag property of each ListViewItem object inside your ListView control and reference it later when trying to find the existing product.
For example, your code could be modified as such :

private void buttonAddItem_Click(object sender, EventArgs e)
{
   int productID = yourCurrentProductID; // which can be saved in the listBoxItems.SelectedItem.Tag as well
   ListViewItem existingItem = null;
   foreach (ListViewItem item in listViewOrders.Items)
   {
     if ((int)item.Tag == productID)
     {
      existingItem = item;
      break;
     }
   }

   if (existingItem == null)
   { // the item is not already in the list, lets add it
   ListViewItem pItem = new ListViewItem(listBoxItems.SelectedItem.ToString());

     pItem.Tag = productID;

     ListViewItem.ListViewSubItem sItem1 = new ListViewItem.ListViewSubItem(pItem, Convert.ToString(QuantityForm.quantity));
     ListViewItem.ListViewSubItem sItem2 =
          new ListViewItem.ListViewSubItem(pItem, Convert.ToString(QuantityForm.quantity * Convert.ToDouble(labelItemPrice.Text)));
     pItem.SubItems.Add(sItem1);
     pItem.SubItems.Add(sItem2);
     listViewOrders.Items.Add(pItem); }
   }
   else
   { // the item already exists in the list
     existingItem.SubItems[1].Text = Convert.ToString(QuantityForm.quantity + Convert.ToInt32(existingItem.SubItems[1].Text));
     existingItem.SubItems[2].Text =
          Convert.ToString((QuantityForm.quantity * Convert.ToDouble(labelItemPrice.Text)) + Convert.ToDouble(existingItem.SubItems[2].Text));
   }

Hope this helps!
Simon