How Do I Make A Duplicate File Finder

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;
using System.IO;

namespace WindowsFormsApplication2
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        OpenFileDialog ofd = new OpenFileDialog();
        FolderBrowserDialog fbd = new FolderBrowserDialog();

        private void button1_Click(object sender, EventArgs e)
        {
            
            ofd.Multiselect = true;
            if (ofd.ShowDialog() == DialogResult.OK)
            {
                string[] fileNameAndPath = ofd.FileNames;
                string[] filename = ofd.SafeFileNames;
                for (int i = 0; i < ofd.SafeFileNames.Count(); i++)
                {
                    listView1.Items.Add(filename[i] );
                }
            }
        }

     

        private void button2_Click(object sender, EventArgs e)
        {
            if (fbd.ShowDialog() == DialogResult.OK)
            {
                


                string foldername = fbd.SelectedPath;
                foreach (string f in Directory.GetFiles(foldername))
                    listView1.Items.Add(Path.GetFileName((f)));
                    

            }
        }

        private void button3_Click(object sender, EventArgs e)
        {
            listView1.Items.Clear();
        }

        private void button4_Click(object sender, EventArgs e)
        {
            listView1.Items.Remove(listView1.SelectedItems[0]);

        }

        private void exitToolStripMenuItem_Click(object sender, EventArgs e)
        {
            Application.Exit();
        }

        private void infoToolStripMenuItem_Click(object sender, EventArgs e)
        {
            MessageBox.Show("lalala");
        }

        private void button5_Click(object sender, EventArgs e)
        {
            for (int i = 0; i < System.Windows.Form.listView1; i++)
            {
            }
        }       
    }
}