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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

Callback,
} from 'shared/ReactSideEffectTags';
import getComponentName from 'shared/getComponentName';
import invariant from 'shared/invariant';
import warningWithoutStack from 'shared/warningWithoutStack';
import {NoWork, Sync} from './ReactFiberExpirationTime';
import {onCommitUnmount} from './ReactFiberDevToolsHook';
import {startPhaseTimer, stopPhaseTimer} from './ReactDebugFiberPerf';
import {getStackByFiberInDevAndProd} from './ReactCurrentFiber';
import {logCapturedError} from './ReactFiberErrorLogger';
import {getCommitTime} from './ReactProfilerTimer';
import {commitUpdateQueue} from './ReactUpdateQueue';
import {
  getPublicInstance,
  supportsMutation,
  supportsPersistence,
  commitMount,
  commitUpdate,
  resetTextContent,
  commitTextUpdate,
  appendChild,
  appendChildToContainer,
  insertBefore,
  insertInContainerBefore,
  removeChild,
  removeChildFromContainer,
  replaceContainerChildren,
  createContainerChildSet,
  hideInstance,
  hideTextInstance,