Save text on my.settings saved multiple times vb.net - vb.net

Question

i have start a new application to encrypt strings and save them on my settings the code its work fine but wen it save it save multiple times the same.
How can i do for save all the content from a listbox to my settings with out to repeat.
This is my code
Imports System.Text
Imports System.Security.Cryptography
Imports System
Imports System.Collections
Imports System.Collections.Specialized
Public Class Form1
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
'My.Settings.md5_hashes.Clear()
End Sub
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim md5 As MD5 = System.Security.Cryptography.MD5.Create()
Dim inputBytes As Byte() = System.Text.Encoding.ASCII.GetBytes(TextBox1.Text)
Dim hash As Byte() = md5.ComputeHash(inputBytes)
Dim sb As New StringBuilder()
For i As Integer = 0 To hash.Length - 1
sb.Append(hash(i).ToString("x2"))
Next
TextBox2.Text = sb.ToString
ListBox1.Items.Add(TextBox1.Text + "<--->" + TextBox2.Text)
My.Settings.md5_hashes.Add(TextBox1.Text + "<--->" + TextBox2.Text)
My.Settings.Save()
End Sub
Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
For Each item In My.Settings.md5_hashes
ListBox1.Items.Add(item)
Next
Timer1.Start()
End Sub
Private Sub Timer1_Tick(sender As Object, e As EventArgs) Handles Timer1.Tick
For Each item In My.Settings.md5_hashes
ListBox1.Items.Add(item)
Next
End Sub
Private Sub Button3_Click(sender As Object, e As EventArgs) Handles Button3.Click
TextBox1.Text = ""
TextBox2.Text = ""
End Sub
End Class

Answer 1

Well i just solved my self the problem its because wen i start the timer its is there the code to retrieved all the content from my settings to the listbox.
I just removed and solved.