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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

return (
        <Suspense fallback={<Text text="Loading..." />}>
          <ErrorBoundary ref={errorBoundary}>
            <AsyncText text="Result" ms={1000} />
          </ErrorBoundary>
        </Suspense>
      );
    }
    ReactNoop.render(<App />);
    expect(ReactNoop.flush()).toEqual(['Suspend! [Result]', 'Loading...']);
    expect(ReactNoop.getChildren()).toEqual([]);
    textResourceShouldFail = true;
    ReactNoop.expire(1000);
    await advanceTimers(1000);
    textResourceShouldFail = false;
    expect(ReactNoop.flush()).toEqual([
      'Promise rejected [Result]',
      'Error! [Result]',
      
      'Error! [Result]',
      
      'Caught error: Failed to load: Result',
    ]);
    expect(ReactNoop.getChildren()).toEqual([
      span('Caught error: Failed to load: Result'),
    ]);
  });
  it('retries on error after falling back to a placeholder', async () => {
    class ErrorBoundary extends React.Component {