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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

state = {error: null};
                  static getDerivedStateFromError(error) {
                    return {error};
                  }
                  render() {
                    advanceTimeBy(2);
                    return this.state.error === null ? (
                      this.props.children
                    ) : (
                      <AdvanceTime byAmount={20} />
                    );
                  }
                }
                advanceTimeBy(5); 
                ReactTestRenderer.create(
                  <React.unstable_Profiler id="test" onRender={callback}>
                    <ErrorBoundary>
                      <AdvanceTime byAmount={5} />
                      <ThrowsError />
                    </ErrorBoundary>
                  </React.unstable_Profiler>,
                );
                expect(callback).toHaveBeenCalledTimes(1);
                
                let [mountCall] = callback.mock.calls;
                
                
                expect(mountCall[1]).toBe('mount');