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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

rootWithPendingPassiveEffects = null;
  passiveEffectCallbackHandle = null;
  passiveEffectCallback = null;
  
  const previousIsRendering = isRendering;
  isRendering = true;
  let effect = firstEffect;
  do {
    if (effect.effectTag & Passive) {
      let didError = false;
      let error;
      if (__DEV__) {
        invokeGuardedCallback(null, commitPassiveHookEffects, null, effect);
        if (hasCaughtError()) {
          didError = true;
          error = clearCaughtError();
        }
      } else {
        try {
          commitPassiveHookEffects(effect);
        } catch (e) {
          didError = true;
          error = e;
        }
      }
      if (didError) {
        captureCommitPhaseError(effect, error);
      }
    }
    effect = effect.nextEffect;