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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

let timeoutPropMs = workInProgress.pendingProps.maxDuration;
        if (typeof timeoutPropMs === 'number') {
          if (timeoutPropMs <= 0) {
            earliestTimeoutMs = 0;
          } else if (
            earliestTimeoutMs === -1 ||
            timeoutPropMs < earliestTimeoutMs
          ) {
            earliestTimeoutMs = timeoutPropMs;
          }
        }
      }
      workInProgress = workInProgress.return;
    } while (workInProgress !== null);
    
    workInProgress = returnFiber;
    do {
      if (
        workInProgress.tag === SuspenseComponent &&
        shouldCaptureSuspense(workInProgress.alternate, workInProgress)
      ) {
        
        
        
        const pingTime =
          (workInProgress.mode & ConcurrentMode) === NoEffect
            ? Sync
            : renderExpirationTime;
        
        let onResolveOrReject = retrySuspendedRoot.bind(