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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

reconcileChildFibers(
          workInProgress,
          currentFallbackChild,
          null,
          renderExpirationTime,
        );
         
        child = next = primaryChild;
      }
    } else {
       
       
      const currentPrimaryChild: Fiber = (current.child: any);
      if (nextDidTimeout) {
         
         
        const nextFallbackChildren = nextProps.fallback;
        const primaryChildFragment = createFiberFromFragment(
           
           
          null,
          mode,
          NoWork,
          null,
        );
        primaryChildFragment.effectTag |= Placement;
        primaryChildFragment.child = currentPrimaryChild;
        currentPrimaryChild.return = primaryChildFragment;
         
        const fallbackChildFragment = (primaryChildFragment.sibling = createFiberFromFragment(