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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

workLoop(isYieldy);
    } catch (thrownValue) {
      resetContextDependences();
      resetHooks();
      if (nextUnitOfWork === null) {
        // 
        didFatal = true;
        onUncaughtError(thrownValue);
      } else {
        if (__DEV__) {
          // 
          // 
          (resetCurrentlyProcessingQueue: any)();
        }
        const failedUnitOfWork: Fiber = nextUnitOfWork;
        if (__DEV__ && replayFailedUnitOfWorkWithInvokeGuardedCallback) {
          replayUnitOfWork(failedUnitOfWork, thrownValue, isYieldy);
        }
        // 
        // 
        // 
        invariant(
          nextUnitOfWork !== null,
          'Failed to replay rendering after an error. This ' +
            'is likely caused by a bug in React. Please file an issue ' +
            'with a reproducing case to help us find it.',
        );
        const sourceFiber: Fiber = nextUnitOfWork;
        let returnFiber = sourceFiber.return;
        if (returnFiber === null) {