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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

Container,
  ChildSet,
} from './ReactFiberHostConfig';
import type {SuspenseState} from './ReactFiberSuspenseComponent';
import {
  IndeterminateComponent,
  FunctionComponent,
  ClassComponent,
  HostRoot,
  HostComponent,
  HostText,
  HostPortal,
  ContextProvider,
  ContextConsumer,
  ForwardRef,
  Fragment,
  Mode,
  Profiler,
  SuspenseComponent,
  MemoComponent,
  SimpleMemoComponent,
  LazyComponent,
  IncompleteClassComponent,
} from 'shared/ReactWorkTags';
import {Placement, Ref, Update} from 'shared/ReactSideEffectTags';
import invariant from 'shared/invariant';
import {
  createInstance,
  createTextInstance,
  createHiddenTextInstance,