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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

expect(ReactTestRenderer).toHaveYielded(['Yield:5']);
          
          
          expect(callback).toHaveBeenCalledTimes(1);
          let call = callback.mock.calls[0];
          expect(call[2]).toBe(5); 
          expect(call[3]).toBe(5); 
          expect(call[4]).toBe(115); 
          expect(call[5]).toBe(120); 
          callback.mockReset();
          
          expect(renderer).toFlushWithoutYielding();
          expect(callback).toHaveBeenCalledTimes(0);
        });
        it('should report the expected times when a high-priority update replaces a low-priority update', () => {
          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={6} />
              <Yield renderTime={15} />
            </React.unstable_Profiler>,
            {unstable_isConcurrent: true},
          );