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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

enableProfilerTimer = true,
  enableSchedulerTracing = true,
  replayFailedUnitOfWorkWithInvokeGuardedCallback = false,
  useNoopRenderer = false,
} = {}) {
  let currentTime = 0;
  mockNow = jest.fn().mockImplementation(() => currentTime);
  global.Date.now = mockNow;
  ReactFeatureFlags = require('shared/ReactFeatureFlags');
  ReactFeatureFlags.debugRenderPhaseSideEffects = false;
  ReactFeatureFlags.debugRenderPhaseSideEffectsForStrictMode = false;
  ReactFeatureFlags.enableProfilerTimer = enableProfilerTimer;
  ReactFeatureFlags.enableSchedulerTracing = enableSchedulerTracing;
  ReactFeatureFlags.replayFailedUnitOfWorkWithInvokeGuardedCallback = replayFailedUnitOfWorkWithInvokeGuardedCallback;
  React = require('react');
  SchedulerTracing = require('scheduler/tracing');
  ReactCache = require('react-cache');
  if (useNoopRenderer) {
    ReactNoop = require('react-noop-renderer');
    ReactTestRenderer = null;
  } else {
    ReactNoop = null;
    ReactTestRenderer = require('react-test-renderer');
    ReactTestRenderer.unstable_setNowImplementation(mockNow);
  }