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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

this.componentDidCatch(error, {
        componentStack: stack !== null ? stack : '',
      });
      if (__DEV__) {
        if (typeof getDerivedStateFromError !== 'function') {
          
          
          
          warningWithoutStack(
            fiber.expirationTime === Sync,
            '%s: Error boundaries should implement getDerivedStateFromError(). ' +
              'In that method, return a state update to display an error message or fallback UI.',
            getComponentName(fiber.type) || 'Unknown',
          );
        }
      }
    };
  }
  return update;
}
function throwException(
  root: FiberRoot,
  returnFiber: Fiber,
  sourceFiber: Fiber,
  value: mixed,
  renderExpirationTime: ExpirationTime,
) {
  
  sourceFiber.effectTag |= Incomplete;