title: Prevent duplicates from adding items from Listbox1 to Listbox2 (VBA excel)
body: <p>I have two ListBoxes. ListBox1 has list of items that can be selected by the user to transfer to ListBox2 by either double clicking the item or pressing the add button. What I want to do now is to prevent the user from adding duplicates in ListBox2. If ever a duplicate is detected a message will prompt "Item already included" and end the code. I am guessing this can be done with contains? But I have no idea how to do it. I have the following codes:</p>
  
  <pre><code>'Report Listing
  Private Sub UserForm_Initialize()
      'List of Reports
      With ListBox1
          .AddItem "Report 1"
          .AddItem "Report 2"
          .AddItem "Report 3"
          .AddItem "Report 4"
          .AddItem "Report 5"
          .AddItem "Report 6"
      End With
  End Sub
  
  'Add selection to ListBox2
  Private Sub AddButton_Click()
  
          With ListBox1
          Dim itemIndex As Integer
          For itemIndex = .ListCount - 1 To 0 Step -1
              If .Selected(itemIndex) Then
                  ListBox2.AddItem .List(itemIndex)
              End If
          Next itemIndex
      End With
  
  End Sub
  
      'Double click to Add
  Private Sub ListBox1_DblClick(ByVal Cancel As MSForms.ReturnBoolean)
      ListBox2.AddItem ListBox1.List(ListBox1.ListIndex)
  End Sub
  </code></pre>