Luhn’s algorithm

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

////namespace parta

namespace LuhnCardValidate
{
    public partial class Frmcard : Form
    {
        public Frmcard()
        {
            InitializeComponent();
        }
        public static bool validateluhn(String cardNumber);
        public void button1_Click(object sender, EventArgs e)
        {
            if (button1.Enabled == true)
            {
                card();
            }
        }

        public void card()
        {
            string cardnumber = "5191 7011 9192 1874";

            if (validateluhn(cardnumber))
            {//changed because have to use form not console though cant test to see if what im doing works!
                lblvalid.Text = ("Valid"); //was console.WriteLine. . .
                this.Text = txtnum.Text; //was console.Read();
            }
            else
            {
                Console.WriteLine("Not Valid");
                Console.Read();
            }

        }

        public static bool validateluhn(String cardNumber)
        {

            //Clean the card number- remove dashes and spaces
            // Hint use the Replace method for this

            //Convert card number into digits array

            int[] digits = new int[cardnumber.Length];


            for (int len = 0; len < cardnumber.Length; len++)
            {
                digits[len] = Int32.Parse(cardNumber.Substring(len, 1));
            }

            //Luhn's Algorithm
            //Hans Peter Luhn (July 1, 1896 – August 19, 1964)

            int sum = 0;
            bool alt = false;
            for (int i = digits.Length - 1; i >= 0; i--)
            {
                int currentDigit = digits[i];
                if (alt)
                {
                    currentDigit *= 2;
                    if (currentDigit > 9)
                    {
                        currentDigit -= 9;
                    }
                }
                sum += currentDigit;
                alt = !alt;
            }

            //If Mod 10 equals 0, the number is good and this will return true
            return sum % 10 == 0;

        }
    }


theres an error sayin that the parta.frmcard already defines a member called validateluhhn with the same parameter types is this because the way i have saved my work? any help greatly many thanks