Because checking the ledger and claiming the right to issue a refund are different operations. If two workers can both read “not refunded” before either records a claim, both may proceed. A second common possibility is that a provider accepted the first request but the response was lost, so a retry created a new logical request. The title alone does not establish which happened.
Preventing a repeat takes two layers: atomically reserve each intended refund in your own durable state, and use the same provider idempotency key when retrying that same refund. Then reconcile uncertain outcomes against provider responses and webhooks rather than treating a timeout as proof that nothing happened.
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How can a ledger check pass and still allow two refunds?
A read-before-write check is not a concurrency control mechanism. Consider two workers handling the same payment at nearly the same time:
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- Worker A reads the ledger and sees no refund.
- Worker B reads the ledger before A has committed anything and also sees no refund.
- Both submit a refund request.
Each read can be correct when it happens, yet the combined result can be wrong. The check did not reserve the refund or prevent another worker from making the same decision.
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A related failure can occur without simultaneous workers: the provider accepts a refund, but the response is lost or times out. If the application retries as a new operation, it may submit a second request. A timeout means the outcome is unknown, not necessarily that the provider did nothing.
These are plausible failure modes for the incident described, not a confirmed diagnosis. Without the handler, database behavior, provider, and event timeline, it is not possible to identify the actual cause.
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What should the handler do instead?
1. Give each intended refund a stable identity
Create an internal identifier for the logical refund, tied to the original payment and the intended amount and reason. A retry of that same intended refund must keep the same identity; a genuinely separate refund should have a different one.
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2. Claim the refund atomically in durable state
In one durable operation, validate the refundable balance and reserve or claim the logical refund. Use a transaction, locking strategy, uniqueness constraint, or an equivalent guarantee appropriate to your database and architecture. The important property is that concurrent workers cannot both successfully claim the same action based on a stale “not refunded” read.
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Do not mark the refund as completed merely because it was claimed. Keep distinct states such as claimed, submitted, pending or unknown, succeeded, and failed, so a claim is not mistaken for a provider-confirmed outcome.
3. Send a stable provider idempotency key
Use a key derived from the internal refund identity, and reuse it for retries of the same logical operation. Keep the request parameters consistent across those retries. Stripe documents that it saves the first status code and response body once endpoint execution begins, compares parameters for a reused key, and may prune keys once they are at least 24 hours old. Adyen recommends UUID-style keys and documents a validity period of 7–14 days at company-account scope. These behaviors are provider-specific; verify the contract for the API and version you use. See Stripe’s idempotent requests documentation and Adyen’s API idempotency documentation.
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4. Treat timeouts and pending results as unresolved
Do not generate a fresh key or submit a fresh logical refund just because the client did not receive a response. Retry with the same key where the provider’s rules allow it, and preserve the request’s identity and parameters. Record the provider request or refund reference and its current state so the operation can be reconciled rather than duplicated.
5. Process webhooks idempotently and reconcile
Where the provider reports an asynchronous outcome, consume the webhook in a way that safely handles repeated delivery. Match it to the refund request and original payment, update local state, and compare provider refund records with your ledger. Adyen’s refund guidance describes waiting for the refund outcome webhook; its referenced-refund flow links a refund to the original payment’s PSP reference and provides an audit trail across full and partial refunds. See Adyen’s refund guidance and referenced refunds.
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What do provider safeguards cover—and what do they not cover?
Provider-side amount limits are useful protections, but they do not replace your local concurrency control or establish why a particular handler duplicated an operation.
| Behavior | Stripe | Adyen |
|---|---|---|
| Idempotency | POST requests accept idempotency keys. Stripe saves the first result after endpoint execution begins and compares parameters on reuse. Keys may be pruned once at least 24 hours old. Stripe documentation | POST requests support keys; UUID-style keys are recommended. The documented validity is 7–14 days at company-account scope. Adyen documentation |
| Concurrent or ambiguous requests | A same-key concurrent conflict may not save an idempotent result if endpoint execution did not begin. Follow the specific endpoint and error response. Stripe documentation | A documented same-key race can produce a transient error; retry later with the same key only when the transient-error signal allows it. Adyen documentation |
| Refund amount guard | A refund cannot exceed the remaining unrefunded amount; a fully refunded charge cannot be refunded again. Stripe documentation | Default accounting rules prevent total refunds from exceeding the captured amount. Adyen documentation |
| Refund tracking | A refund is created against a Charge or PaymentIntent; partial refunds are supported up to the remaining amount. Stripe documentation | The refund response returns a PSP reference, and the outcome is reported through a webhook. Referenced refunds use the original payment reference. Refund guidance and referenced refunds |
These mechanisms operate at different layers. A provider’s maximum-amount rule can reject excess value, for example, but the handler still needs to prevent duplicate logical work, preserve operation identity, and resolve uncertain outcomes. Stripe also documents HTTP 409 conflict cases and recommends exponential backoff for HTTP 429 rate limiting; neither status should be treated as a universal signal that every request is safe to retry. Follow the documented semantics for the specific error and endpoint. See Stripe’s errors documentation.
How should you investigate an actual duplicate?
- Compare local refund identities. Determine whether two provider requests came from one logical refund retried with a new identity, or from two workers claiming the same intended refund.
- Reconstruct request timing. Compare ledger reads and writes, worker or job IDs, outbound request timestamps, timeouts, retries, and provider responses. Overlapping reads before a claim is committed support a race hypothesis; a lost response followed by a new key supports an ambiguous-retry hypothesis.
- Match provider records to local records. Use the provider refund or request reference and the original payment identifier. Check whether webhooks arrived late, more than once, or not at all before the handler retried.
- Compare amounts and remaining balance. Establish whether the provider accepted both operations and whether either was partial. Amount caps may limit total damage, but do not establish the handler’s root cause.
- Review retry and error handling. Confirm that retries preserve the same idempotency key and parameters, and that conflict, rate-limit, transient, and timeout outcomes are handled according to the provider’s specific contract.
When is an asynchronous reversal endpoint appropriate?
Adyen’s Checkout v71 reversal endpoint is an endpoint-specific asynchronous alternative: it returns a unique request reference and reports the outcome through a CANCEL_OR_REFUND webhook. Its documentation cautions against using this endpoint for payments with multiple partial captures. It is not a universal refund recipe; use it only when its payment and capture conditions match your integration. See the Checkout v71 reversal endpoint.
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