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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

[true, false].forEach(
          replayFailedUnitOfWorkWithInvokeGuardedCallback => {
            describe(`replayFailedUnitOfWorkWithInvokeGuardedCallback ${
              replayFailedUnitOfWorkWithInvokeGuardedCallback
                ? 'enabled'
                : 'disabled'
            }`, () => {
              beforeEach(() => {
                jest.resetModules();
                loadModules({
                  replayFailedUnitOfWorkWithInvokeGuardedCallback,
                });
              });
              it('should accumulate actual time after an error handled by componentDidCatch()', () => {
                const callback = jest.fn();
                const ThrowsError = () => {
                  advanceTimeBy(3);
                  throw Error('expected error');
                };
                class ErrorBoundary extends React.Component {
                  state = {error: null};
                  componentDidCatch(error) {
                    this.setState({error});
                  }
                  render() {
                    advanceTimeBy(2);
                    return this.state.error === null ? (
                      this.props.children
                    ) : (
                      <AdvanceTime byAmount={20} />