What validation rule can I use on a form to ensure that data entered into the form does not duplicate a previous entry? Ive tried using the
"DLookup ("CustomerID", "Customers", "CustomerID = Forms!Customers!CustomerID") Is Null"
command suggested by Access Help but cannot get it to work. Anybody any ideas?

In your table, you can indicate that the field should be Indexed with No Duplicates.