Prevent Duplicate value entering into the Database

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

namespace WindowsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {

        }

        private void button1_Click(object sender, EventArgs e)
        {
            string name = textBox1.Text;

            string pword = textBox2.Text;
            System.Data.OleDb.OleDbConnection conn = new
             System.Data.OleDb.OleDbConnection();

            conn.ConnectionString = @"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=C:\data1\jana.mdb";
            conn.Open();
            System.Data.OleDb.OleDbDataAdapter dr = new System.Data.OleDb.OleDbDataAdapter("select * from mans1", conn);
            
            DataSet s = new DataSet();
            //dr.SelectCommand = comm;
            dr.Fill(s);

            foreach (DataRow dar in s.Tables[0].Rows)
            {
                textBox3.Text = dar[1].ToString();
            }
            int k = 0;
            foreach (DataRow darr in s.Tables[0].Rows)
            {
               if (name == darr[1].ToString())
                {
                   MessageBox.Show("code already exist");
                    k = 1;
                    break;
                }
            }
            if (k == 0)
            {
                string insertstatement = "INSERT INTO mans1(name,pword) VALUES( '" + textBox1.Text + "','" + textBox2.Text + "')";

                System.Data.OleDb.OleDbCommand cmd = new System.Data.OleDb.OleDbCommand(insertstatement, conn);

                int n = cmd.ExecuteNonQuery();

                if (n > 0)
                {
                   MessageBox.Show("code saved");
                }
            }
            else
            {
                MessageBox.Show("error");
            }
            
            conn.Close();

            Form1.ActiveForm.Hide();
            Form2 a = new Form2();
            a.Show();
        }

        private void button2_Click(object sender, EventArgs e)
        {

        }
    }
}