Duplicate

I have a form with a bunch of fields. Now their are three im concered with. CFR_EHPID,CFR_SVCDATE,CFR_PROVTAXID. If thier is a vaule in the database when the user tries to enter it again, i want a dup message to show...... SO i dont want any duplicates for those three feilds. is their a way when the user is done keying and tries to move to the next record it displays that thier is a duplicate??????. below is the code i use but its for the before update event



Dim SID As String
Dim stLinkCriteria As String
Dim rsc As DAO.Recordset
Set rsc = Me.RecordsetClone
SID = Me.CFR_EHPID.Value
stLinkCriteria = "[CFR_EHPID]=" & "'" & SID & "'"
'Check StudentDetails table for duplicate StudentNumber
If DCount("CFR_EHPID", "CFR", stLinkCriteria) > 0 Then

'Undo duplicate entry

'Message box warning of duplication
MsgBox "Warning Student Number " _
& SID & " has already been entered." _
& vbCr & vbCr & "You will now been taken to the record.", vbInformation _
, "Duplicate Information"
'Go to record of original Student Number

End If
Set rsc = Nothing
End Sub