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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

} while (effect !== null);
  isRendering = previousIsRendering;
  // Check if work was scheduled by one of the effects
  const rootExpirationTime = root.expirationTime;
  if (rootExpirationTime !== NoWork) {
    requestWork(root, rootExpirationTime);
  }
}
function isAlreadyFailedLegacyErrorBoundary(instance: mixed): boolean {
  return (
    legacyErrorBoundariesThatAlreadyFailed !== null &&
    legacyErrorBoundariesThatAlreadyFailed.has(instance)
  );
}
function markLegacyErrorBoundaryAsFailed(instance: mixed) {
  if (legacyErrorBoundariesThatAlreadyFailed === null) {
    legacyErrorBoundariesThatAlreadyFailed = new Set([instance]);
  } else {
    legacyErrorBoundariesThatAlreadyFailed.add(instance);
  }
}
function flushPassiveEffects() {
  if (passiveEffectCallback !== null) {
    Schedule_cancelCallback(passiveEffectCallbackHandle);
    
    
    passiveEffectCallback();
  }
}
function commitRoot(root: FiberRoot, finishedWork: Fiber): void {