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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

if (!supportsPersistence) {
    return;
  }
  switch (finishedWork.tag) {
    case ClassComponent: {
      return;
    }
    case HostComponent: {
      return;
    }
    case HostText: {
      return;
    }
    case HostRoot:
    case HostPortal: {
      const portalOrRoot: {
        containerInfo: Container,
        pendingChildren: ChildSet,
      } =
        finishedWork.stateNode;
      const {containerInfo, pendingChildren} = portalOrRoot;
      replaceContainerChildren(containerInfo, pendingChildren);
      return;
    }
    default: {