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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(renderer).toFlushAndYield(['Yield:6', 'Yield:15']);
          expect(callback).toHaveBeenCalledTimes(1);
          let call = callback.mock.calls[0];
          expect(call[2]).toBe(21); 
          expect(call[3]).toBe(21); 
          expect(call[4]).toBe(5); 
          expect(call[5]).toBe(26); 
          callback.mockReset();
          advanceTimeBy(30); 
          
          
          renderer.update(
            <React.unstable_Profiler id="test" onRender={callback}>
              <Yield renderTime={3} />
              <Yield renderTime={5} />
              <Yield renderTime={9} />
            </React.unstable_Profiler>,
          );
          expect(renderer).toFlushAndYieldThrough(['Yield:3']);
          expect(callback).toHaveBeenCalledTimes(0);
          
          advanceTimeBy(100); 
          
          expect(renderer).toFlushAndYieldThrough(['Yield:5']);
          expect(callback).toHaveBeenCalledTimes(0);
          
          advanceTimeBy(100);