It able to update to the database but it pop up error message like this "Violation of PRIMARY KEY constraint 'PK_ProductStock_1'. Cannot insert duplicate key in object 'dbo.ProductStock.' The statement has been terminated."

Your help will be appreciated

Thx you

if (CheckBoxSub8XS.Checked == true && txtSub8XS.Text != "")
            {
                using (SqlConnection conStrstock8 = new SqlConnection(ConfigurationManager.ConnectionStrings["ConnectionString"].ToString()))
                {

                    string sqlupStock8 = "UPDATE ProductStock SET ProductQty=@qty1 WHERE ProductID=@pid AND ProductColorType=@protType AND ProductSize=@size";

                    using (SqlCommand cmdupstock8 = new SqlCommand(sqlupStock8, conStrstock8))
                    {
                        cmdupstock8.Parameters.AddWithValue("@size", " XS");
                        cmdupstock8.Parameters.AddWithValue("@qty1", txtSub8XS.Text);
                        cmdupstock8.Parameters.AddWithValue("@protType", "Type8");
                        cmdupstock8.Parameters.AddWithValue("@pid", Id);

                        conStrstock8.Open();
                        cmdupstock8.ExecuteNonQuery();
                        conStrstock8.Close();

                    }
                }
         }

if (CheckBoxSub8XS.Checked == true)
            {
                using (SqlConnection conINstock8 = new SqlConnection(ConfigurationManager.ConnectionStrings["ConnectionString"].ToString()))
                {

                    string sqlINStock8 = "INSERT INTO ProductStock(ProductID,ProductColorType,ProductSize,ProductQty,ProductSizeEntre,ProductSizeQty) VALUES(@pid,@protType,@size,@qty,@sizeEnt,@sizeQty)";

                    using (SqlCommand cmdINstock8 = new SqlCommand(sqlINStock8, conINstock8))
                    {
                        cmdINstock8.Parameters.AddWithValue("@pid", Id);
                        cmdINstock8.Parameters.AddWithValue("@protType", "Type8");
                        cmdINstock8.Parameters.AddWithValue("@size", " XS");
                        cmdINstock8.Parameters.AddWithValue("@qty", txtSub8XS.Text);
                        cmdINstock8.Parameters.AddWithValue("@sizeEnt", " XS");
                        cmdINstock8.Parameters.AddWithValue("@sizeQty", txtSub8XS.Text);

                        conINstock8.Open();
                        cmdINstock8.ExecuteNonQuery();
                        conINstock8.Close();

                    }
                }
            }