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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

getCommitTime(),
          );
        }
      }
      return;
    }
    case SuspenseComponent: {
      if (finishedWork.effectTag & Callback) {
        
        
        
        
        const newState: SuspenseState = {
          alreadyCaptured: true,
          didTimeout: false,
          timedOutAt: NoWork,
        };
        finishedWork.memoizedState = newState;
        flushPassiveEffects();
        scheduleWork(finishedWork, Sync);
        return;
      }
      let oldState: SuspenseState | null =
        current !== null ? current.memoizedState : null;
      let newState: SuspenseState | null = finishedWork.memoizedState;
      let oldDidTimeout = oldState !== null ? oldState.didTimeout : false;
      let newDidTimeout;
      let primaryChildParent = finishedWork;
      if (newState === null) {
        newDidTimeout = false;