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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

NoEffect,
  ShouldCapture,
  Callback as CallbackEffect,
  LifecycleEffectMask,
} from 'shared/ReactSideEffectTags';
import {enableSchedulerTracing} from 'shared/ReactFeatureFlags';
import {ConcurrentMode} from './ReactTypeOfMode';
import {shouldCaptureSuspense} from './ReactFiberSuspenseComponent';
import {createCapturedValue} from './ReactCapturedValue';
import {
  enqueueCapturedUpdate,
  createUpdate,
  CaptureUpdate,
} from './ReactUpdateQueue';
import {logError} from './ReactFiberCommitWork';
import {getStackByFiberInDevAndProd} from './ReactCurrentFiber';
import {popHostContainer, popHostContext} from './ReactFiberHostContext';
import {
  isContextProvider as isLegacyContextProvider,
  popContext as popLegacyContext,
  popTopLevelContextObject as popTopLevelLegacyContextObject,
} from './ReactFiberContext';
import {popProvider} from './ReactFiberNewContext';
import {
  renderDidSuspend,
  renderDidError,
  onUncaughtError,
  markLegacyErrorBoundaryAsFailed,
  isAlreadyFailedLegacyErrorBoundary,
  retrySuspendedRoot,