DropDownList duplicates when added to Cart

// fill the control with data
    private void PopulateControls(ProductDetails pd)
    {
            // display product recommendations
        string productId = pd.ProductId.ToString();
        recommendations.LoadProductRecommendations(productId);
   
            // display the product details
        titleLabel.Text = pd.Name;
        descriptionLabel.Text = pd.Description;
        priceLabel.Text = String.Format("{0:c}", pd.Price);
        productImage.ImageUrl = "ProductImages/" + pd.Image2FileName;
            // set the title of the page
        this.Title = BalloonShopConfiguration.SiteName + pd.Name;

            // obtain the attributes of the product
        DataTable attrTable = CatalogAccess.GetProductAttributes(productId);

            // temp variables
        string prevAttributeName = "";
        string attributeName, attributeValue, attributeValueId;

            // current DropDown for attribute values
        Label attributeNameLabel;
        DropDownList attributeValuesDropDown = new DropDownList();

            // read the list of attributes
        foreach (DataRow r in attrTable.Rows)
        {
                // get attribute data
            attributeName = r["AttributeName"].ToString();
            attributeValue = r["AttributeValue"].ToString();
            attributeValueId = r["AttributeValueID"].ToString();

                // if starting a new attribute (e.g. Color, Size)
            if (attributeName != prevAttributeName)
            {
                prevAttributeName = attributeName;
                attributeNameLabel = new Label();
                attributeNameLabel.Text = attributeName + ": ";
                attributeValuesDropDown = new DropDownList();
                attrPlaceHolder.Controls.Add(attributeNameLabel);
                attrPlaceHolder.Controls.Add(attributeValuesDropDown);
            }
                // add a new attribute value to the DropDownList
            attributeValuesDropDown.Items.Add(new ListItem(attributeValue, attributeValueId));
        }
    }

    protected void AddToCartButton_Click(object sender, EventArgs e)
    {
            // Retrieve ProductID from the query string
        string productId = Request.QueryString["ProductID"];

            // Retrieve the selected product options
        string options = "";
        foreach (Control cnt in attrPlaceHolder.Controls)
        {
            if (cnt is Label)
            {
                Label attrLabel = (Label)cnt;
                options += attrLabel.Text;
            }

            if (cnt is DropDownList)
            {
                DropDownList attrDropDown = (DropDownList)cnt;
                options = attrDropDown.Items[attrDropDown.SelectedIndex].Value;
            }
        }

        // Add the product to the shopping cart
        ShoppingCartAccess.AddItem(productId, options);

    }