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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

id: 0,
          name: 'initial render',
          timestamp: mockNow(),
        };
        const onRender = jest.fn();
        let renderer;
        SchedulerTracing.unstable_trace(
          interaction.name,
          interaction.timestamp,
          () => {
            renderer = ReactTestRenderer.create(
              <React.unstable_Profiler id="app" onRender={onRender}>
                <React.Suspense
                  maxDuration={2000}
                  fallback={<Text text="loading" />}>
                  <AsyncText text="loaded" ms={1000} />
                </React.Suspense>
              </React.unstable_Profiler>,
              {unstable_isConcurrent: true},
            );
          },
        );
        renderer.unstable_flushAll();
        expect(onInteractionScheduledWorkCompleted).not.toHaveBeenCalled();
        jest.advanceTimersByTime(1000);
        await resourcePromise;
        renderer.unstable_flushAll();
        expect(onInteractionScheduledWorkCompleted).toHaveBeenCalledTimes(1);
        expect(
          onInteractionScheduledWorkCompleted,