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**Do you want to request a *feature* or report a *bug*?**
Not sure if its a bug or a feature request.

**What is the current behavior?**
The component given as a fallback to Suspense is remounted each time a lazy child resolves.

https://codesandbox.io/s/z6v6x3n1np
This example shows how a fallback component, which counts up, is mounted more than once.

**What is the expected behavior?**
The fallback is mounted once and is only unmounted when all lazy children are resolved.
In the example the counter would run up to 9.

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**
react@16.6.0

}
      if ((effect.tag & mountTag) !== NoHookEffect) {
        
        const create = effect.create;
        let destroy = create();
        if (typeof destroy !== 'function') {
          if (__DEV__) {
            if (destroy !== null && destroy !== undefined) {
              warningWithoutStack(
                false,
                'useEffect function must return a cleanup function or ' +
                  'nothing.%s%s',
                typeof destroy.then === 'function'
                  ? ' Promises and useEffect(async () => ...) are not ' +
                    'supported, but you can call an async function inside an ' +
                    'effect.'
                  : '',
                getStackByFiberInDevAndProd(finishedWork),
              );
            }
          }
          destroy = null;
        }
        effect.destroy = destroy;
      }
      effect = effect.next;
    } while (effect !== firstEffect);
  }
}
export function commitPassiveHookEffects(finishedWork: Fiber): void {