If Len(str_to_add) > 0 Then
      SC = ListView1.ListItems.count
      On Error Resume Next
      Set lItem = ListView1.ListItems.Add(, "key" & str_to_add, SC + 1)
      If Err.NUMBER = 0 Then
        ListViewSetItemPicture ListView1, lItem.index, 0
        lItem.SubItems(1) = str_to_add
      End If
    End If
    FormProgress.IncreasePValue
  Next i

...
        If Me.ListView1.Items("key" & str_to_add) Is Nothing Then 'проверка отсутствия "ключа"
          LItem = Me.ListView1.Items.Add(New ListViewItem)
          LItem.Name = "key" & str_to_add
          LItem.Text = (SC + 1).ToString
          LItem.ImageKey = "NUMBERS"
          LItem.SubItems.Add(New ListViewItem.ListViewSubItem) 'добавим пустой SubItem -1шт
          LItem.SubItems(1).Text = str_to_add
        Else
          'Debug.Print("exist")
        End If
      End If
    Next

   Dim coll As New Collection
    For Each item As ListViewItem In ListView1.Items
      coll.Add(Nothing, item.Name)
    Next

    For...
...
      If Strings.Len(str_to_add) > 0 Then
        SC = Me.ListView1.Items.Count
        'проверка что таких данных не существует под другими записями
        If coll.Contains("key" & str_to_add) = False Then
           LItem = Me.ListView1.Items.Add(New ListViewItem)
          LItem.Name = "key" & str_to_add
          LItem.Text = (SC + 1).ToString
          LItem.ImageKey = "NUMBERS"
          LItem.SubItems.Add(New ListViewItem.ListViewSubItem) 'добавим пустой SubItem -1шт
          LItem.SubItems(1).Text = str_to_add
          coll.Add(Nothing, LItem.Name)
        Else
          'Debug.Print("exist")
        End If
      End If
    Next
    coll.Clear()