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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

instance = this;
          return (
            <React.Fragment>
              <Yield value="first" />
              {this.state.count}
              <Yield value="last" />
            </React.Fragment>
          );
        }
      }
      advanceTimeBy(1);
      const interactionCreation = {
        id: 0,
        name: 'creation event',
        timestamp: mockNow(),
      };
      const onRender = jest.fn();
      let renderer;
      SchedulerTracing.unstable_trace(
        interactionCreation.name,
        mockNow(),
        () => {
          renderer = ReactTestRenderer.create(
            <React.unstable_Profiler id="test-profiler" onRender={onRender}>
              <Example />
            </React.unstable_Profiler>,
            {
              unstable_isConcurrent: true,
            },