We have a couple of cases where paypal has transacted 2 (e.g) chargebacks against the same donation with different amounts. (they seem to add up to the original). In these cases they have different paypal IDs (but our order ID/ the gravy IDs are the same) - I think we can use handling similar to the way we do this with the payments - ie treat as separate if the backend_processor_reversal_id differs