Delete specific selection in multiple selection Drop Down list in VB

[Duette]

I've been trying to find a way to use the MID and LEN functions in VB. I have an Excel 2003 spreadsheet that uses multiple dropdown selections placed in one cell. What I would like to do is every time the user accidentally selects something that has already been put in the cell it returns nothing, as opposed to placing a duplicate in the cell. When there are duplicates there's nothing you can do but delete the entire cell and start over with your selections. Here's my code:

Option Explicit
Private Sub Worksheet_Change(ByVal Target As Range)
Dim rngDV As Range
Dim oldVal As String
Dim newVal As String
If Target.Count > 1 Then GoTo exitHandler
On Error Resume Next
Set rngDV = Cells.SpecialCells(xlCellTypeAllValidation)
On Error GoTo exitHandler
If rngDV Is Nothing Then GoTo exitHandler
If Intersect(Target, rngDV) Is Nothing Then
   'do nothing
Else
  Application.EnableEvents = False
  newVal = Target.Value
  Application.Undo
  oldVal = Target.Value
  Target.Value = newVal
  If Target.Column = 3 Then
    If oldVal = "" Then
      'do nothing
      ElseIf newVal = Application.WorksheetFunction.Mid(oldVal, _
      Application.WorksheetFunction.Find(newVal, oldVal), _
      Application.WorksheetFunction.Len(oldVal)) Then
      'do nothing
      ElseIf newVal = "" Then
      'do nothing
      Else
      Target.Value = oldVal & "," & Chr(10) & newVal
      End If
  End If
End If
exitHandler:
  Application.EnableEvents = True
End Sub

The line of code that gets tripped is bolded. Any suggestions?