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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

popTopLevelLegacyContextObject(workInProgress);
      const fiberRoot = (workInProgress.stateNode: FiberRoot);
      if (fiberRoot.pendingContext) {
        fiberRoot.context = fiberRoot.pendingContext;
        fiberRoot.pendingContext = null;
      }
      if (current === null || current.child === null) {
        
        
        popHydrationState(workInProgress);
        
        
        workInProgress.effectTag &= ~Placement;
      }
      updateHostContainer(workInProgress);
      break;
    }
    case HostComponent: {
      popHostContext(workInProgress);
      const rootContainerInstance = getRootHostContainer();
      const type = workInProgress.type;
      if (current !== null && workInProgress.stateNode != null) {
        updateHostComponent(
          current,
          workInProgress,
          type,
          newProps,
          rootContainerInstance,
        );
        if (current.ref !== workInProgress.ref) {