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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 nextRenderExpirationTime: ExpirationTime = NoWork;
let nextLatestAbsoluteTimeoutMs: number = -1;
let nextRenderDidError: boolean = false;
let nextEffect: Fiber | null = null;
let isCommitting: boolean = false;
let rootWithPendingPassiveEffects: FiberRoot | null = null;
let passiveEffectCallbackHandle: * = null;
let passiveEffectCallback: * = null;
let legacyErrorBoundariesThatAlreadyFailed: Set<mixed> | null = null;
// Used for performance tracking.
let interruptedBy: Fiber | null = null;
let stashedWorkInProgressProperties;
let replayUnitOfWork;
let isReplayingFailedUnitOfWork;
let originalReplayError;
let rethrowOriginalError;
if (__DEV__ && replayFailedUnitOfWorkWithInvokeGuardedCallback) {
  stashedWorkInProgressProperties = null;
  isReplayingFailedUnitOfWork = false;
  originalReplayError = null;
  replayUnitOfWork = (
    failedUnitOfWork: Fiber,
    thrownValue: mixed,
    isYieldy: boolean,
  ) => {
    if (
      thrownValue !== null &&
      typeof thrownValue === 'object' &&
      typeof thrownValue.then === 'function'
    ) {