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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

state = {count: 0};
        render() {
          second = this;
          ReactTestRenderer.unstable_yield('SecondComponent');
          return null;
        }
      }
      advanceTimeBy(5);
      const renderer = ReactTestRenderer.create(
        <React.unstable_Profiler id="test" onRender={onRender}>
          <FirstComponent />
          <SecondComponent />
        </React.unstable_Profiler>,
        {unstable_isConcurrent: true},
      );
      
      renderer.unstable_flushAll(['FirstComponent', 'SecondComponent']);
      expect(onInteractionTraced).not.toHaveBeenCalled();
      expect(onInteractionScheduledWorkCompleted).not.toHaveBeenCalled();
      onRender.mockClear();
      advanceTimeBy(100);
      const interactionLowPri = {
        id: 0,
        name: 'lowPri',
        timestamp: mockNow(),
      };
      SchedulerTracing.unstable_trace(interactionLowPri.name, mockNow(), () => {
        
        first.setState({count: 1});