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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

import type {FunctionComponentUpdateQueue} from './ReactFiberHooks';
import {
  enableHooks,
  enableSchedulerTracing,
  enableProfilerTimer,
} from 'shared/ReactFeatureFlags';
import {
  FunctionComponent,
  ForwardRef,
  ClassComponent,
  HostRoot,
  HostComponent,
  HostText,
  HostPortal,
  Profiler,
  SuspenseComponent,
  IncompleteClassComponent,
  MemoComponent,
  SimpleMemoComponent,
} from 'shared/ReactWorkTags';
import {
  invokeGuardedCallback,
  hasCaughtError,
  clearCaughtError,
} from 'shared/ReactErrorUtils';
import {
  ContentReset,
  Placement,
  Snapshot,
  Update,