Hi
This is a classic problem we all come across eventually.
Add the items to a collection first, then add those to the list

Private Sub cmb1_Click()
  myRow = ActiveCell.Row
  Dim Bcell As Range
  Dim ListCollection as New Collection, ListValue as Variant, item as
Variant
  Form1.ListBox1.Clear
On error resume next
For Each Bcell In Range(("A" & myRow) & (":J" & myRow))
    If Bcell.Value = "C" Or Bcell.Value = "S" Or Bcell.Value = "I"
Then
        ListValue =
ThisWorkbook.Sheets("Sheet1").Range(((ConvertToLetter(Bcell.Column)) &
"1"))
        ListCollection.Add ListValue, Cstr(ListValue)
    End if
Next Bcell
on error goto 0
'now add to listbox
    for each Item in ListCollection
      Form1.ListBox1.AddItem Item
   next item
end sub

The add in a collection creates an error if the item already exists,
and the copy is not added. The "on error" bits catch the errors.
Check John Walkenbach's website for more on this (like sorting the
list) - I think this was his idea first.

regards
Paul