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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,
  enableUserTimingAPI,
  replayFailedUnitOfWorkWithInvokeGuardedCallback,
  warnAboutDeprecatedLifecycles,
} from 'shared/ReactFeatureFlags';
import getComponentName from 'shared/getComponentName';
import invariant from 'shared/invariant';
import warningWithoutStack from 'shared/warningWithoutStack';
import ReactFiberInstrumentation from './ReactFiberInstrumentation';
import * as ReactCurrentFiber from './ReactCurrentFiber';
import {
  now,
  scheduleDeferredCallback,
  cancelDeferredCallback,
  shouldYield,
  prepareForCommit,
  resetAfterCommit,
  scheduleTimeout,
  cancelTimeout,
  noTimeout,
} from './ReactFiberHostConfig';
import {
  markPendingPriorityLevel,
  markCommittedPriorityLevels,
  markSuspendedPriorityLevel,
  markPingedPriorityLevel,
  hasLowerPriorityWork,
  isPriorityLevelSuspended,
  findEarliestOutstandingPriorityLevel,
  didExpireAtExpirationTime,