Why does React Strict Mode repeat renders and Effects in development?

Uses development-only repeated rendering, Effects, and ref callbacks to expose impurity and missing cleanup.

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TL;DR

Strict Mode deliberately repeats some development-only work, including rendering components and rerunning Effects and ref callbacks. This exposes impure rendering and setup code that lacks matching cleanup. It does not mean production performs every operation twice. I fix the component so a setup-cleanup-setup sequence is safe instead of adding a flag that suppresses the second call.

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Practice focus

Strict Mode behavior, render purity, effect symmetry, ref cleanup, and production-safe diagnosis.

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Core answer
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Overview
Repeated development work is a diagnostic signal, not a reason to disable the safety check or hide a lifecycle bug.
Strict Mode deliberately repeats some development-only work, including rendering components and rerunning Effects and ref callbacks. This exposes impure rendering and setup code that lacks matching cleanup. It does not mean production performs every operation twice. I fix the component so a setup-cleanup-setup sequence is safe instead of adding a flag that suppresses the second call.
Checkpoint
Stronger answer
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I first identify what repeated: component calculation, an Effect, a callback ref, or an event the application actually fired twice. Rendered code must not mutate props, append to shared arrays, or create externally visible work. An Effect that opens a connection, observer, timer, or subscription must close exactly that resource in cleanup. A development duplicate network request may be acceptable if it is abortable and harmless, but a purchase or other command belongs behind a user event and an idempotent server boundary.

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Common mistakesCandidateCommon mistakesMistakes that weaken the answer or signal shallow understanding.Open details

Details

Disabling Strict Mode as the first fix.
Using a didRun ref to hide missing cleanup.
Putting consequential mutations in a mount Effect.

Why does React Strict Mode repeat renders and Effects in development?

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