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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

current: Fiber | null,
  workInProgress: Fiber,
  Component: any,
  shouldUpdate: boolean,
  hasContext: boolean,
  renderExpirationTime: ExpirationTime,
) {
  
  markRef(current, workInProgress);
  const didCaptureError = (workInProgress.effectTag & DidCapture) !== NoEffect;
  if (!shouldUpdate && !didCaptureError) {
    
    if (hasContext) {
      invalidateContextProvider(workInProgress, Component, false);
    }
    return bailoutOnAlreadyFinishedWork(
      current,
      workInProgress,
      renderExpirationTime,
    );
  }
  const instance = workInProgress.stateNode;
  
  ReactCurrentOwner.current = workInProgress;
  let nextChildren;