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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

it('should report the expected times when a high-pri update replaces a mount in-progress', () => {
          const callback = jest.fn();
          const Yield = ({renderTime}) => {
            advanceTimeBy(renderTime);
            ReactTestRenderer.unstable_yield('Yield:' + renderTime);
            return null;
          };
          advanceTimeBy(5); 
          
          
          const renderer = ReactTestRenderer.create(
            <React.unstable_Profiler id="test" onRender={callback}>
              <Yield renderTime={10} />
              <Yield renderTime={20} />
            </React.unstable_Profiler>,
            {unstable_isConcurrent: true},
          );
          expect(renderer).toFlushAndYieldThrough(['Yield:10']);
          expect(callback).toHaveBeenCalledTimes(0);
          
          advanceTimeBy(100); 
          
          
          renderer.unstable_flushSync(() => {
            renderer.update(
              <React.unstable_Profiler id="test" onRender={callback}>
                <Yield renderTime={5} />
              </React.unstable_Profiler>,
            );
          });