Prevent Duplicate Entries VBA - vba

Question

First off I am a bit of a novice when its comes to VBA, so everything I do is a bit of hit and miss but normally I eventually figure out the problem.
However this time I have been stuck for days and can't seem to find the issue!
I have the following form and subforms with the below structure. (Access2013)
Main Form [Job Number]
Subform [Out2] (this is where a user scans a barcode into the relevant field)
Subform [DS] (this is where the scanned barcode from [Out2] creates a new record)
Subform [DS] fields : Id, Job No, BarCode, Description, Date, User
What I am trying to achieve with the code below, is in 'The Before Update' event of the [DS] BarCode field, the Dcount function will check the list of Barcodes already entered in the subform container [DS], and if there
is more than one it will undo the duplicate entry. Unfortunately nothing is happening when a duplicate entry is entered.
(not even errors)
P.S. Setting the table (No Duplicates) thing will not work for this DB.
Private Sub BarCode_BeforeUpdate(Cancel As Integer)
Dim BarCode As String
Dim strLinkCriteria As String
Dim rsc As DAO.Recordset
Set rsc = Me.RecordsetClone
BarCode = Me.BarCode.Text
strLinkCriteria = "[Barcode]=" & "'" & Replace(Me![BarCode], "'", "''")
'Check Items Subform for duplicate BarCode
If DCount("BarCode", "Forms![Job Number]![DS]", strLinkCriteria) > 0 Then
'Undo duplicate entry
Me.Undo
'Message box warning of duplication
MsgBox "Warning Item Title " _
& BarCode & " has already been entered." _
& vbCr & vbCr & "You will now been taken to the record.", _
vbInformation, "Duplicate Information"
'Go to record of original Title
rsc.FindFirst strLinkCriteria
Me.Bookmark = rsc.Bookmark
End If
Set rsc = Nothing
End Sub

Answer 1

Here is how to handle this:
Private Sub BarCode_BeforeUpdate(Cancel As Integer)
Dim rsc As DAO.Recordset
Dim BarCode As String
Dim Criteria As String
Set rsc = Me.RecordsetClone
BarCode = Nz(Me!BarCode.Value)
Criteria = "[Barcode] = '" & Replace(BarCode, "'", "''") & "'")
rsc.FindFirst Criteria
Cancel = Not rsc.NoMatch
If Cancel = True Then
' Message box warning of duplication
MsgBox "Warning Item Title " _
& BarCode & " has already been entered." _
& vbCrLf & vbCrLf & "You will now been taken to the record.", _
vbInformation, "Duplicate Information"
' Go to record of original Title
Me.Bookmark = rsc.Bookmark
End If
Set rsc = Nothing
End Sub