Preventing Duplicate Values before the save

I found a piece of coding that I would like to use to prevent duplicate values before the record is a saved.
The table that I created is called "tblCWSHIPDIRECT". The primary key is called "reff". The primary key value is a concatenate value using several fields in the table tblCWSHIPDIRECT, so the value will always be unique.  Give you ideal what I'm doing, code below is what I use to save the information. In the code snippet box, I 'm trying apply logic to the coding  that before the save record can be save,  check the txtref agasint reff table field for duplicate values. If none exist, save reocrd, if it casues   I want the record to be save, if does create a duplicate value, prompt the user a error message that record can't be saved because it will cause a duplicate value.

' If everything is ok, save record
   Set rst = db.OpenRecordset _
      ("SELECT * FROM tblCWSHIPDIRECT", dbOpenDynaset, dbSeeChanges)
   ' Add a new record
   rst.AddNew
   ' Specify the values for the fields
   rst!Reff = Me.txtRef
   rst!CreateDate = Me.datCreateDate
   rst!Loc = Me.txtloc
   rst!LocName = Me.txtLocation
   rst!LofPart = Me.txtlofpart
   rst!Barcode = Me.txtbarcode
   rst!TotalPallets = Me.nbrtotalpallet
   rst!PalletQty = Me.nbrpalletqty
   rst!OrderQty = Me.nbrpalletqty
   rst!TrailerNbr = Me.nbrtrailernbr
   ' Save the changes you made to the
   ' current record in the Recordset
   rst.Update
   

Me.datCreateDate = Date
Me.txtRef = ""
Me.txtloc = Null
Me.txtLocation = Null
Me.txtlofpart = Null
Me.txtbarcode = Null
Me.nbrtotalpallet = Null
Me.nbrpalletqty = Null
Me.nbrqty = Null


MsgBox "Record Saved"

rst.Close.  

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Open a connection.
   ' Open the database
   Set db = CurrentDb()
   ' Open the Recordset
   
'Check for duplicate Reference values
 strSQL = "SELECT tblCWSHIPDIRECT.Reff, FROM tblCWSHIPDIRECT "
 strSQL = strSQL & "WHERE (((tblCWSHIPDIRECT.Reff) In (SELECT [Me.txtref] FROM [tblCWSHIPDIRECT] As Tmp "
 strSQL = strSQL & "GROUP BY [Me.txtref] HAVING Count(*)>1)));"
 Set db = CurrentDb()
  Set rst2 = db.OpenRecordset(strSQL)
 
 
  If rst.BOF And rst.EOF Then
    MsgBox "No records to process"
  Else
    rst.MoveFirst
    Do Until rst.EOF
      strDupValue = rst.Fields(0)
      If strDupValue = strSaveValue Then
        rst.Delete
      Else
        strSaveValue = rst.Fields(0)
      End If
      rst.MoveNext
    Loop
   
    Set rst = Nothing
    Set db = Nothing
   
  End If