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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

sourceFiber.firstEffect = sourceFiber.lastEffect = null;
  if (
    value !== null &&
    typeof value === 'object' &&
    typeof value.then === 'function'
  ) {
    
    const thenable: Thenable = (value: any);
    
    
    
    
    
    let workInProgress = returnFiber;
    let earliestTimeoutMs = -1;
    let startTimeMs = -1;
    do {
      if (workInProgress.tag === SuspenseComponent) {
        const current = workInProgress.alternate;
        if (current !== null) {
          const currentState: SuspenseState | null = current.memoizedState;
          if (currentState !== null && currentState.didTimeout) {
            
            
            const timedOutAt = currentState.timedOutAt;
            startTimeMs = expirationTimeToMs(timedOutAt);
            
            break;
          }
        }