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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

let React;
let ReactTestRenderer;
let ReactFeatureFlags;
let ReactCache;
let Suspense;
let TextResource;
let textResourceShouldFail;
describe('ReactSuspense', () => {
  beforeEach(() => {
    jest.resetModules();
    ReactFeatureFlags = require('shared/ReactFeatureFlags');
    ReactFeatureFlags.debugRenderPhaseSideEffectsForStrictMode = false;
    ReactFeatureFlags.replayFailedUnitOfWorkWithInvokeGuardedCallback = false;
    ReactFeatureFlags.enableHooks = true;
    React = require('react');
    ReactTestRenderer = require('react-test-renderer');
    
    ReactCache = require('react-cache');
    Suspense = React.Suspense;
    TextResource = ReactCache.unstable_createResource(([text, ms = 0]) => {
      let listeners = null;
      let status = 'pending';
      let value = null;
      return {
        then(resolve, reject) {