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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 maxDuration={500} fallback={<Text text="Loading..." />}>
          <Text text="A" />
          <AsyncText ms={1000} text="B" />
          <Text text="C" />
        </Suspense>
      );
    }
    ReactDOM.render(<App />, container);
    expect(container.textContent).toEqual('Loading...');
    await advanceTimers(1000);
    expect(container.textContent).toEqual('ABC');
  });
});