Sub CarregaCliente(cbo As ComboBox)
   Dim rsCbo As ADODB.Recordset
  
   cbo.Clear
   Set rsCbo = DB.Execute("SELECT Codigo FROm Clientes Group by Codigo ORDER BY Codigo")
   Do While Not rsCbo.EOF
      cbo.AddItem rsCbo!Codigo
      rsCbo.MoveNext
   Loop ' rsCbo
   rsCbo.Close
'Voce esta usando o "Group By"
End Sub

No load do form coloque

CarregaCliente Combo1