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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

} from './ReactFiberExpirationTime';
import {ConcurrentMode, ProfileMode, NoContext} from './ReactTypeOfMode';
import {
  enqueueUpdate,
  resetCurrentlyProcessingQueue,
  ForceUpdate,
  createUpdate,
} from './ReactUpdateQueue';
import {createCapturedValue} from './ReactCapturedValue';
import {
  isContextProvider as isLegacyContextProvider,
  popTopLevelContextObject as popTopLevelLegacyContextObject,
  popContext as popLegacyContext,
} from './ReactFiberContext';
import {popProvider, resetContextDependences} from './ReactFiberNewContext';
import {resetHooks} from './ReactFiberHooks';
import {popHostContext, popHostContainer} from './ReactFiberHostContext';
import {
  recordCommitTime,
  startProfilerTimer,
  stopProfilerTimerIfRunningAndRecordDelta,
} from './ReactProfilerTimer';
import {
  checkThatStackIsEmpty,
  resetStackAfterFatalErrorInDev,
} from './ReactFiberStack';
import {beginWork} from './ReactFiberBeginWork';
import {completeWork} from './ReactFiberCompleteWork';
import {
  throwException,