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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

if (__DEV__) {
      ReactCurrentFiber.setCurrentFiber(workInProgress);
    }
    const returnFiber = workInProgress.return;
    const siblingFiber = workInProgress.sibling;
    if ((workInProgress.effectTag & Incomplete) === NoEffect) {
      // 
      if (enableProfilerTimer) {
        if (workInProgress.mode & ProfileMode) {
          startProfilerTimer(workInProgress);
        }
        nextUnitOfWork = completeWork(
          current,
          workInProgress,
          nextRenderExpirationTime,
        );
        if (workInProgress.mode & ProfileMode) {
          // 
          stopProfilerTimerIfRunningAndRecordDelta(workInProgress, false);
        }
      } else {
        nextUnitOfWork = completeWork(
          current,
          workInProgress,
          nextRenderExpirationTime,
        );
      }
      stopWorkTimer(workInProgress);
      resetChildExpirationTime(workInProgress, nextRenderExpirationTime);
      if (__DEV__) {