Duplicate Zustand store subscribers after React 18 Strict Mode remount

Direct answer

Yes. With React 18 Strict Mode in development, Zustand's subscription cleanup runs after the simulated unmount and before the remount. Each component instance ends the mount/unmount/remount cycle with zero net subscribers from that instance. This behavior is the same in Zustand 4.x and 5.x.

Established behavior and likely explanation

Established: Strict Mode double-mount only occurs when process.env.NODE_ENV !== 'production'. It does not run in production builds, so duplicate listeners from Strict Mode cannot reach production.

Likely explanation: extra subscriptions outside Strict Mode usually come from an unstable selector reference, such as useStore(state => state.value). A new function reference on every render creates a new subscription, which looks like duplicate listeners but is selector instability, not a cleanup failure.

Steps for this migration

1. Use a stable selector. Prefer useShallow(state => state.field) in Zustand 4.x, or memoize the selector with useCallback.
2. Verify cleanup in development only. Render the component under <React.StrictMode> and confirm the console shows subscribe, unsubscribe, then subscribe.
3. Run a production build with NODE_ENV=production and confirm no double-mount sequence appears.

Observing subscribers safely

Do not access store._subscribers; it is private and may change. Add a temporary middleware that increments a counter on subscribe and decrements it on unsubscribe, then expose getSubscriberCount. Remove it before production if bundle size is a concern.

Assumption: you are using React 18 with Zustand 4.x or 5.x. If duplicate subscribers appear in production, share whether the selector reference changes between renders; that detail determines whether the fix is memoization or a store-level issue.