Do these 3 things before closing this tab:
1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesIf this is a React app, a development-only duplicate request may come from Strict Mode deliberately replaying an Effect once to expose missing cleanup. But Strict Mode is not the explanation for every duplicate: Effects can rerun when dependencies change, and production duplicates require investigation. The right fix depends on whether the request is a read, a connection, or a user-triggered write.
Why an API request may run twice in React
With root-level <StrictMode>, React runs an extra Effect setup-and-cleanup cycle in development before the first real setup. React describes the check this way: “When Strict Mode is on, React will run one extra setup+cleanup cycle before the first real setup.” The check is development-only; React’s documentation says it does not affect production builds. React Strict Mode documentation
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This replay can make a request in an Effect appear twice in the browser’s Network panel. It is a diagnostic, not a guarantee that every duplicate is caused by Strict Mode. React also runs an Effect again when its dependencies change. An actual unmount and remount—for example, because routing or application state changes—can also run the Effect for a new component instance. React: Synchronizing with Effects
How to tell what is causing the duplicate
- Compare development with production. Check the same interaction in a production build. If the extra request occurs only in development, Strict Mode replay is a plausible cause; if it also occurs in production, investigate other triggers.
- Check the Strict Mode boundary. Look for root-level
<StrictMode>in the React app’s entry point. Its presence can explain a development-only Effect replay, but does not prove that it caused every repeated request. - Inspect the Effect dependencies. Check whether a value in the dependency array changes after the first render. A changed dependency is a normal reason for React to run the Effect again.
- Check for a real remount. Look at route changes and state-driven conditional rendering that may remove and recreate the component.
- Separate the request from other network activity. In the browser’s Network panel, inspect which interaction triggered each request and whether both requests have the same URL, method, and payload.
Make Effect cleanup match setup
An Effect that starts an external process should return cleanup that stops or reverses that process. For example, disconnect a connection or unsubscribe from a listener when the component’s Effect is cleaned up. This lets React test whether setup and cleanup correctly mirror each other. React: Synchronizing with Effects
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Cleanup is not a way to undo a request the server has already received or processed. Aborting or otherwise stopping client-side work may prevent some further client activity, but it cannot promise to reverse a server-side action that has already happened.
For data reads, use caching or request deduplication
For fetching data to display, a request cache or data-fetching layer can deduplicate equivalent requests or reuse cached responses where appropriate. React’s documentation identifies request deduplication and caching as capabilities a data-fetching solution can provide; it does not prescribe a particular library on the cited page. React: Fetching data with Effects
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That approach addresses repeated reads more directly than trying to make an Effect run only once. An Effect’s lifecycle can be replayed, and dependencies or remounts may legitimately trigger it again. A cache can help keep those lifecycle events from turning into unnecessary equivalent network requests.
For user-triggered writes, send the request from the event handler
Requests that change data because a person took an action—such as buying an item, submitting a payment, or creating a record—belong in the click or form-submit handler. They should not be triggered by an Effect that is meant to synchronize with rendering. React’s guidance distinguishes interaction-driven work from Effects that synchronize with external systems. React: You Might Not Need an Effect
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Moving a write to the event handler ties it to the user’s action rather than to the component’s mounting lifecycle. It does not, by itself, make retries safe: a timeout can leave the client unsure whether the server completed the operation.
Use server-supported idempotency for uncertain retries
When a write may need to be retried after a timeout or an uncertain response, use the API provider’s documented idempotency mechanism if it offers one. Stripe documents idempotency keys as a way to retry requests safely without accidentally performing the same operation twice. Follow the provider’s own rules for key use and retention; Stripe’s behavior should not be assumed to apply to other APIs. Stripe: Idempotent requests
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The HTTP Idempotency-Key header is described by MDN, but the cited IETF specification is an Internet-Draft, not a finalized standard. Its status and the server’s actual behavior matter: a header alone does not make an API idempotent unless the API supports and documents it. MDN: Idempotency-Key IETF HTTPAPI Idempotency-Key draft
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| What you see | Best next step |
|---|---|
| Development-only extra request from an Effect | Check root-level Strict Mode and make the Effect’s cleanup mirror its setup. |
| Repeated data read | Check dependency changes and remounts; consider request caching or deduplication. |
| Duplicate in production | Inspect dependencies, user interactions, routing, remounts, and request triggers rather than blaming Strict Mode. |
| Write triggered by a click or submit | Put it in the event handler; use the API provider’s idempotency support for uncertain retries. |
Disabling Strict Mode does not fix a missing cleanup, a dependency-driven rerun, a real remount, or an unsafe retry. Keep the diagnostic enabled and address the cause that the Network panel and component lifecycle reveal.
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