Changing a ListBox through its own eventhandler

If you could, post your code please.  I wrote a sample program to do what you mentioned, and I didn't witness what you've said.  Here's what I've written.

Public Class Form1
    Inherits System.Windows.Forms.Form

#Region " Windows Form Designer generated code "

    Public Sub New()
        MyBase.New()

        'This call is required by the Windows Form Designer.
        InitializeComponent()

        'Add any initialization after the InitializeComponent() call

    End Sub

    'Form overrides dispose to clean up the component list.
    Protected Overloads Overrides Sub Dispose(ByVal disposing As Boolean)
        If disposing Then
            If Not (components Is Nothing) Then
                components.Dispose()
            End If
        End If
        MyBase.Dispose(disposing)
    End Sub

    'Required by the Windows Form Designer
    Private components As System.ComponentModel.IContainer

    'NOTE: The following procedure is required by the Windows Form Designer
    'It can be modified using the Windows Form Designer.  
    'Do not modify it using the code editor.
    Friend WithEvents Timer1 As System.Windows.Forms.Timer
    Friend WithEvents Button1 As System.Windows.Forms.Button
    Friend WithEvents ListBox1 As System.Windows.Forms.ListBox
    <System.Diagnostics.DebuggerStepThrough()> Private Sub InitializeComponent()
        Me.components = New System.ComponentModel.Container
        Me.Timer1 = New System.Windows.Forms.Timer(Me.components)
        Me.Button1 = New System.Windows.Forms.Button
        Me.ListBox1 = New System.Windows.Forms.ListBox
        Me.SuspendLayout()
        '
        'Timer1
        '
        Me.Timer1.Interval = 1000
        '
        'Button1
        '
        Me.Button1.Location = New System.Drawing.Point(48, 224)
        Me.Button1.Name = "Button1"
        Me.Button1.Size = New System.Drawing.Size(72, 24)
        Me.Button1.TabIndex = 1
        Me.Button1.Text = "Button1"
        '
        'ListBox1
        '
        Me.ListBox1.Items.AddRange(New Object() {"Item1", "Item2", "Item3", "Item4"})
        Me.ListBox1.Location = New System.Drawing.Point(16, 8)
        Me.ListBox1.Name = "ListBox1"
        Me.ListBox1.Size = New System.Drawing.Size(120, 173)
        Me.ListBox1.TabIndex = 2
        '
        'Form1
        '
        Me.AutoScaleBaseSize = New System.Drawing.Size(5, 13)
        Me.ClientSize = New System.Drawing.Size(292, 273)
        Me.Controls.Add(Me.ListBox1)
        Me.Controls.Add(Me.Button1)
        Me.Name = "Form1"
        Me.Text = "Form1"
        Me.ResumeLayout(False)

    End Sub

#End Region

    Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles MyBase.Load

    End Sub

    Private Sub ListBox1_DoubleClick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ListBox1.DoubleClick
        If ListBox1.SelectedIndex = 1 Then
            ListBox1.Items.Clear()
            ListBox1.Items.Add("SubItem1")
            ListBox1.Items.Add("SubItem2")
            ListBox1.Items.Add("SubItem3")
            ListBox1.Items.Add("SubItem4")

        End If
    End Sub
End Class