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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 (!didResolve) {
        ReactTestRenderer.unstable_yield('Suspend!');
        throw thenable;
      }
      ReactTestRenderer.unstable_yield('Async');
      return 'Async';
    }
    const root = ReactTestRenderer.create(
      <Suspense maxDuration={1000} fallback={<Text text="Loading..." />}>
        <Async />
        <Text text="Sibling" />
      </Suspense>,
      {
        unstable_isConcurrent: true,
      },
    );
    expect(root).toFlushAndYieldThrough(['Suspend!']);
    
    resolveThenable();
    expect(ReactTestRenderer).toHaveYielded([]);
    
    expect(root).toFlushAndYield([
      
      'Async',
      'Sibling',
    ]);
    expect(root).toMatchRenderedOutput('AsyncSibling');
  });
  it('mounts a lazy class component in non-concurrent mode', async () => {
    class Class extends React.Component {