title: Multiple ListBox selection avoiding duplicates VBA
body: <p>I am building a Userform which contains two ListBoxes such that the user can select options from ListBox1 and add them to ListBox2 or alternatively remove options from ListBox2</p>
  
  <p>What I am struggling with is how can I prevent the duplicates from being added to the ListBox2? Essentially, I want to build in a function (?) which checks if an option is already included in ListBox2</p>
  
  <pre><code>Private Sub CommandButton3_Click()
  
  '### Adds Items from ListBox1 to ListBox2
  For i = 0 To ListBox1.ListCount - 1
      If ListBox1.Selected(i) = True Then ListBox2.AddItem ListBox1.List(i)
  Next i
  
  ListBox1.Selected
  
  End Sub
  
  Private Sub CommandButton4_Click()
  
  '### Removes Items from ListBox2
  Dim counter As Integer
  counter = 0
  
  For i = 0 To ListBox2.ListCount - 1
      If ListBox2.Selected(i - counter) Then
          ListBox2.RemoveItem (i - counter)
          counter = counter + 1
      End If
  Next i
  
  End Sub
  </code></pre>