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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 (childExpirationTime < renderExpirationTime) {
     
     
     
    return null;
  } else {
     
     
    cloneChildFibers(current, workInProgress);
    return workInProgress.child;
  }
}
function beginWork(
  current: Fiber | null,
  workInProgress: Fiber,
  renderExpirationTime: ExpirationTime,
): Fiber | null {
  const updateExpirationTime = workInProgress.expirationTime;
  if (current !== null) {
    const oldProps = current.memoizedProps;
    const newProps = workInProgress.pendingProps;
    if (
      oldProps === newProps &&
      !hasLegacyContextChanged() &&
      updateExpirationTime < renderExpirationTime
    ) {
       
       
       
      switch (workInProgress.tag) {