How to ignore duplicate data entry in Database in C# using ASP.Net

protected void Btn_GenerateQuotation_Click(object sender, EventArgs e)
        {
           
               if (gridAddProducts.Rows.Count <= 0)
            {
                //this.lblMessage.Visible = true;
            }
            else
            {
                foreach (GridViewRow row in gridAddProducts.Rows)
                {
                    string ProductID = row.Cells[0].Text;
                    string ProductName = row.Cells[1].Text;
                    int Qty = int.Parse(row.Cells[2].Text);
                    decimal Price = decimal.Parse(row.Cells[3].Text);
                    decimal Total = decimal.Parse(row.Cells[4].Text);

                    //this.Save(ProductID,ProductName, Qty, Price, Total);
                    this.Save(ProductID, Qty, Price, Total);
                }
            }
               Btn_PrintQuote.Visible = true;
               Btn_GenerateQuotation.Visible = false;
               
        }
        private void Save(string ProductID, int Qty, decimal Price, decimal Total)
        {
             //if (ListView1.Items.Count == 0)
                //{
                //    MessageBox.Show("sorry no product added", "", MessageBoxButtons.OK, MessageBoxIcon.Warning);
                //    return;
                //}

                auto();
                con = new SqlConnection(cs);
                con.Open();
                string ct = "select QuotationNo from QuotationData where QuotationNo=@find";
                cmd = new SqlCommand(ct);
                cmd.Connection = con;
                cmd.Parameters.Add(new SqlParameter("@find", System.Data.SqlDbType.VarChar, 20, "QuotationNo"));
                cmd.Parameters["@find"].Value = txtQuo_No.Text;
                sdr = cmd.ExecuteReader();
                if (sdr.Read() == true)
                {
                    //MessageBox.Show("Invoice No. Already Exists", "Input Error", MessageBoxButtons.OK, MessageBoxIcon.Error);

                    if ((sdr != null))
                    {
                        sdr.Close();
                    }
                    return;
                }

                con = new SqlConnection(cs);
                con.Open();
                //DateTime QuoDate = new DateTime(txtDate.Value);
                //QuoDate=DateTime.Parse();
                //string cb = "insert Into QuotationData(QuotationNo,QuotationDate,Company,CustomerID,Sales_Person,SubTotal,DiscPercentage,DiscountAmt,GrandTotal,AmtInWords,Payment_Type,QComplete,PComplete,IComplete,RComplete) VALUES ('" + txtQuo_No.Text + "'," + DateTime.Parse(txtDate.Value) + ",'" + ddlCompany.SelectedItem.Text + "'," + txtCus_ID.Text + "," + ddlSalesPerson.SelectedItem.Text + "," + txtSubTotal.Text + "," + txtDis_per.Text + "," + txtDiscount.Text + "," + txtGrandTotal.Text + ",'" + lblword.Text + "','" + ddlPaymentTerms.SelectedItem.Text + "','" + true + "','" + false + "','" + false + "','" + false + "')";
                string cb = "insert Into QuotationData(QuotationNo,QuotationDate,Company,CustomerID,Sales_Person,SubTotal,DiscPercentage,DiscountAmt,GrandTotal,AmtInWords,Payment_Type,QComplete,PComplete,IComplete,RComplete) VALUES (@QuotationNo,@QuotationDate,@Company,@CustomerID,@Sales_Person,@SubTotal,@DiscPercentage,@DiscountAmt,@GrandTotal,@AmtInWords,@Payment_Type,@QComplete,@PComplete,@IComplete,@RComplete)";
                cmd = new SqlCommand(cb);
                cmd.Connection = con;
                cmd.Parameters.AddWithValue("@QuotationNo", txtQuo_No.Text);
                cmd.Parameters.AddWithValue("@QuotationDate",DateTime.Now);
                cmd.Parameters.AddWithValue("@Company",ddlCompany.SelectedItem.Text);
                cmd.Parameters.AddWithValue("@CustomerID",txtCus_ID.Text);
                cmd.Parameters.AddWithValue("@Sales_Person",ddlSalesPerson.SelectedItem.Text);
                cmd.Parameters.AddWithValue("@SubTotal",txtSubTotal.Text);
                cmd.Parameters.AddWithValue("@DiscPercentage",txtDis_per.Text);
                cmd.Parameters.AddWithValue("@DiscountAmt",txtDiscount.Text);
                cmd.Parameters.AddWithValue("@GrandTotal",txtGrandTotal.Text);
                cmd.Parameters.AddWithValue("@AmtInWords",lblword.Text);
                cmd.Parameters.AddWithValue("@Payment_Type",ddlPaymentTerms.SelectedItem.Text);
                cmd.Parameters.AddWithValue("@QComplete",true);
                cmd.Parameters.AddWithValue("@PComplete",false);
                cmd.Parameters.AddWithValue("@IComplete",false);
                cmd.Parameters.AddWithValue("@RComplete",false);
                cmd.ExecuteReader();
                if (con.State == ConnectionState.Open)
                {
                    con.Close();
                }
                con.Close();
                //int index = 0;                
            for(int i=0;i<=gridAddProducts.Rows.Count-1;i++)
            {

            //}
                //for (int i = 0; i <= ListView1.Items.Count - 1; i++)
                //{
                    con = new SqlConnection(cs);

                    string cd = "insert Into QuotationDataDetails(QuotationNo,ProductID,Quantity,Price,TotalAmount) VALUES (@QuotationNo,@ProductID,@Quantity,@Price,@TotalAmount)";
                    cmd = new SqlCommand(cd);
                    cmd.Connection = con;
                    cmd.Parameters.AddWithValue("@QuotationNo", txtQuo_No.Text);
                //cmd.Parameters.AddWithValue("",);
                    cmd.Parameters.AddWithValue("@ProductID", ProductID);               
                    cmd.Parameters.AddWithValue("@Quantity",Qty );
                    cmd.Parameters.AddWithValue("@Price", Price);
                    cmd.Parameters.AddWithValue("@TotalAmount", Total);
                    con.Open();
                    cmd.ExecuteNonQuery();
                    con.Close();
                    //index++;
                }
               
                //GetData();
               // MessageBox.Show("Successfully saved", "Record", MessageBoxButtons.OK, MessageBoxIcon.Information);
        }