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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

ReactNoop.render(
                  <React.unstable_Profiler id="profiler" onRender={jest.fn()}>
                    <errorInCompletePhase>
                      <span>hi</span>
                    </errorInCompletePhase>
                  </React.unstable_Profiler>,
                );
                expect(ReactNoop.flush).toThrow('Error in host config.');
                
                
                ReactNoop.render(
                  <React.unstable_Profiler id="profiler" onRender={jest.fn()}>
                    <span>hi</span>
                  </React.unstable_Profiler>,
                );
                ReactNoop.flush();
              });
            });
          },
        );
      });
      it('reflects the most recently rendered id value', () => {
        const callback = jest.fn();
        advanceTimeBy(5); 
        const renderer = ReactTestRenderer.create(
          <React.unstable_Profiler id="one" onRender={callback}>
            <AdvanceTime byAmount={2} />
          </React.unstable_Profiler>,
        );
        expect(callback).toHaveBeenCalledTimes(1);