You are entering a new record? What would be duplicated - invoice number? Can use domain aggregate function in textbox BeforeUpdate event to check if value already exists in table. Something Like:
If DCount("*", "tablename", "fieldname=" & Me.textboxname) > 0 Then
MsgBox "Invoice number " & Me.textboxname & " already exists."
Cancel = True
Me.textboxName = Null
End If