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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

workInProgress.memoizedProps = workInProgress.pendingProps;
    if (workInProgress.mode & ProfileMode) {
      // 
      stopProfilerTimerIfRunningAndRecordDelta(workInProgress, true);
    }
  } else {
    next = beginWork(current, workInProgress, nextRenderExpirationTime);
    workInProgress.memoizedProps = workInProgress.pendingProps;
  }
  if (__DEV__) {
    ReactCurrentFiber.resetCurrentFiber();
    if (isReplayingFailedUnitOfWork) {
      // 
      // 
      // 
      // 
      rethrowOriginalError();
    }
  }
  if (__DEV__ && ReactFiberInstrumentation.debugTool) {
    ReactFiberInstrumentation.debugTool.onBeginWork(workInProgress);
  }
  if (next === null) {
    // 
    next = completeUnitOfWork(workInProgress);
  }
  ReactCurrentOwner.current = null;
  return next;
}
function workLoop(isYieldy) {