Something like this, perhaps:
Function RemoveDuplicateItems(V As Variant, Delimiter As String) _
As Variant
'Removes duplicate items from a string containing a list of items
'separated by Delimiter.
Dim Uniques As Object 'Scripting.Dictionary
Dim Item As Variant
If IsNull(V) Then
RemoveDuplicateItems = Null
Exit Function
End If
Set Uniques = CreateObject("Scripting.Dictionary")
For Each Item In Split(CStr(V), Delimiter)
If Not Uniques.Exists(Item) Then
Uniques.Add Item, 0
End If
Next
RemoveDuplicateItems = Join(Uniques.Keys, Delimiter)
End Function