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To prevent duplicate charges when a payment agent retries, use two controls at different boundaries: a durable database uniqueness constraint to stop the same event from triggering local work twice, and the payment provider’s idempotency key to make retries of the same remote charge safe. Neither alone guarantees exactly-once processing. You also need a recovery path for a provider success followed by a local failure.
What UNIQUE(event_id) protects—and what it doesn’t
A UNIQUE(event_id) constraint makes the event ID a durable local deduplication key. If the same webhook or queued event arrives again, only one worker can claim that ID in the guarded workflow. Treat a uniqueness conflict as “already claimed,” not as permission to repeat the side effect.
This protects against duplicate deliveries of the same event identity. It does not prove that two different event IDs represent different intended payments. If two events can describe the same business operation, enforce uniqueness on a stable operation-level identifier as well. Nor does a database constraint make a remote payment request and a local database write atomic.
Use a different key for each identity boundary
| Control | Identity it represents | Boundary it protects |
|---|---|---|
Event ledger with UNIQUE(event_id) |
A particular incoming event | Repeated local processing of that event |
| Operation-level key | The intended business action, such as one customer payment | Different events that could otherwise initiate the same action |
| Provider idempotency key | A particular intended API mutation | Repeated attempts to make the remote payment request |
Do not assume a webhook event ID is also the correct payment-operation ID or provider key. Choose the identifier that represents each invariant, persist it, and keep it stable across retries of that same operation. AWS guidance in Idempotency and retries recommends deterministic event IDs; Stripe and Adyen document idempotency for repeated API requests.
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Order the workflow so a retry cannot skip the guard
The uniqueness claim must be durable and made before non-repeatable work begins. A separate “check whether this ID exists, then insert it” sequence is unsafe under concurrency: two workers can both pass the check. Let the database enforce uniqueness as part of the write.
- Derive and persist stable identities. Store the incoming event ID, the business operation ID where needed, and the provider key for the payment attempt. Reuse the same provider key for retries of that mutation.
- Claim the event atomically. Insert the event into a durable ledger protected by a unique constraint. For example, in PostgreSQL-style SQL:
INSERT INTO processed_events (event_id, status) VALUES (:event_id, 'processing') ON CONFLICT (event_id) DO NOTHING RETURNING event_id; - Branch on the claim result. If the insert returns an ID, this worker owns the new claim and may continue. If it returns no row because the ID already exists, do not launch the charge again; respond according to the event source’s duplicate-delivery contract.
- Call the provider with the persisted key. Send the same idempotency key and unchanged request parameters when retrying the same payment. A new key means a new identity to the provider, not “another try” at the old request.
- Record the outcome and advance local work. Persist the provider result and update the workflow state. Guard fulfillment, account crediting, and notifications at their own boundaries; provider idempotency does not deduplicate those local effects.
The SQL is an implementation example, not a cross-database standard. Adapt conflict handling and transaction semantics to your database. Keep network calls out of a database transaction where possible; a transaction cannot make a remote provider call atomic with the local commit.
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Plan explicitly for the uncertain gap
A worker can crash after the provider accepts a charge but before the worker records success. On retry, the local ledger may still say “processing,” while the remote mutation has already happened. Blindly creating a new payment attempt with a fresh key risks a second charge.
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- On an uncertain result, retry the same mutation with the same key while the provider’s contract permits it; do not mint a key for each transport retry.
- Track in-progress work so abandoned claims can be recovered rather than treated as completed or ignored indefinitely.
- Reconcile unresolved attempts against provider results before starting a genuinely new charge.
- Give every follow-on side effect—fulfillment, crediting, or notification—its own duplicate guard or idempotent operation.
This recovery design follows from the separation between local event state and provider request state. A replayed provider result can help resolve the remote attempt, but it does not automatically repair a missing local update.
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Provider idempotency rules differ
Provider contracts are not interchangeable. The operational details below are from current official documentation accessed October 7, 2026; they are provider-specific, not universal standards.
| Provider | Key behavior | Limits and scope |
|---|---|---|
| Stripe | According to Stripe’s Idempotent requests API reference, a repeated request using the same key returns the saved result. Stripe compares request parameters and errors if the key is reused with different parameters. Once endpoint execution begins, the result is saved, including failed status results; validation failures and concurrent execution conflicts are not saved as idempotent results. | Keys may be removed after they are at least 24 hours old, so reusing a pruned key can start a new request. Maximum key length: 255 characters. |
| Adyen | According to Adyen’s API idempotency documentation, POST requests support idempotency. For a timeout, retrying with the same key can avoid duplicating the payment. Adyen says to retry later with the same key when an error is marked transient; a duplicate arriving while the first request is processing may receive a conflict or unprocessable response. | Keys remain valid for 7 to 14 days after first submission. Maximum key length: 64 characters. Keys are stored at company-account level and are not checked for duplicates across regional endpoints. |
These details make expiry, parameter consistency, endpoint scope, and concurrency part of the retry design. Keep a local event ledger for as long as your replay, recovery, and audit needs require; do not treat the provider’s key window as your durable event history.
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Failure modes to test before launch
- Two workers receive one event at once: only one should win the database uniqueness claim; the other must not perform the charge.
- Two event IDs represent one intended payment: event-level uniqueness alone will not catch this; the operation-level invariant must.
- The provider succeeds but the worker times out: retry the same attempt with the same provider key and reconcile its result, rather than issuing a new mutation.
- The same provider key is sent with changed parameters: follow the provider’s documented behavior; Stripe, for example, returns an error for mismatched parameters.
- A key has expired or is outside its scope: the provider may no longer recognize it as the old request. Resolve the old attempt before creating a new one.
- A payment succeeds but fulfillment repeats: provider idempotency covers the provider request, not every downstream action. Add independent guards to those actions.
Stripe’s API reference describes its retry result behavior, and Adyen’s API documentation specifies transient-error and concurrent-request handling. Build retry branches around the actual response contract rather than assuming every timeout, conflict, or failure can be retried identically.
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Design for at-least-once delivery, not exactly once
Retries are necessary because a timeout can leave the caller unable to tell whether a remote operation succeeded. The practical goal is to make repeated attempts safe: deduplicate local event handling, use a stable operation identity for the business invariant, reuse the provider key for the same remote mutation, and reconcile interrupted work. UNIQUE(event_id) is an essential local guard in that design, but it cannot replace the other controls.
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