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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

advanceTimeBy(15); 
        renderer.update(
          <React.unstable_Profiler id="test" onRender={callback}>
            <WithLifecycles />
          </React.unstable_Profiler>,
        );
        expect(callback).toHaveBeenCalledTimes(2);
        const [mountCall, updateCall] = callback.mock.calls;
        expect(mountCall[1]).toBe('mount');
        expect(mountCall[2]).toBe(8); 
        expect(mountCall[3]).toBe(8); 
        expect(mountCall[4]).toBe(5); 
        expect(mountCall[5]).toBe(13); 
        expect(updateCall[1]).toBe('update');
        expect(updateCall[2]).toBe(15); 
        expect(updateCall[3]).toBe(15); 
        expect(updateCall[4]).toBe(28); 
        expect(updateCall[5]).toBe(43); 
      });
      describe('with regard to interruptions', () => {
        it('should accumulate actual time after a scheduling interruptions', () => {
          const callback = jest.fn();
          const Yield = ({renderTime}) => {
            advanceTimeBy(renderTime);
            ReactTestRenderer.unstable_yield('Yield:' + renderTime);
            return null;
          };
          advanceTimeBy(5); 
          
          const renderer = ReactTestRenderer.create(