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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

const prevProps = current.memoizedProps;
          const prevState = current.memoizedState;
          startPhaseTimer(finishedWork, 'componentDidUpdate');
          instance.props = finishedWork.memoizedProps;
          instance.state = finishedWork.memoizedState;
          instance.componentDidUpdate(
            prevProps,
            prevState,
            instance.__reactInternalSnapshotBeforeUpdate,
          );
          stopPhaseTimer();
        }
      }
      const updateQueue = finishedWork.updateQueue;
      if (updateQueue !== null) {
        instance.props = finishedWork.memoizedProps;
        instance.state = finishedWork.memoizedState;
        commitUpdateQueue(
          finishedWork,
          updateQueue,
          instance,
          committedExpirationTime,
        );
      }
      return;
    }
    case HostRoot: {
      const updateQueue = finishedWork.updateQueue;
      if (updateQueue !== null) {
        let instance = null;