Repeat payments from the same payer aren't syncing — what's the setting?

Barclays Business (UK) via GoCardless. When a customer pays the same amount more than once within a few days, only some of those payments come through. Checked against the bank's own statements: about 2.4% of transactions are missing, and they're all repeats of this kind. I've already tried: - Deleting the connection and re-authorising from scratch — identical result. - Setting fields_for_duplicate_filtering to external_identifiers.integrator_transaction_id only, before the backfill ran — no change. - Ruled out pending/booked: every row on this account is booked. What's the correct configuration to stop genuine repeat payments being treated as duplicates?