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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

isWorking = true;
  isCommitting = true;
  startCommitTimer();
  invariant(
    root.current !== finishedWork,
    'Cannot commit the same tree as before. This is probably a bug ' +
      'related to the return field. This error is likely caused by a bug ' +
      'in React. Please file an issue.',
  );
  const committedExpirationTime = root.pendingCommitExpirationTime;
  invariant(
    committedExpirationTime !== NoWork,
    'Cannot commit an incomplete root. This error is likely caused by a ' +
      'bug in React. Please file an issue.',
  );
  root.pendingCommitExpirationTime = NoWork;
  
  
  
  const updateExpirationTimeBeforeCommit = finishedWork.expirationTime;
  const childExpirationTimeBeforeCommit = finishedWork.childExpirationTime;
  const earliestRemainingTimeBeforeCommit =
    childExpirationTimeBeforeCommit > updateExpirationTimeBeforeCommit
      ? childExpirationTimeBeforeCommit
      : updateExpirationTimeBeforeCommit;
  markCommittedPriorityLevels(root, earliestRemainingTimeBeforeCommit);