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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

// 
        next.effectTag &= HostEffectMask;
        return next;
      }
      if (returnFiber !== null) {
        // 
        returnFiber.firstEffect = returnFiber.lastEffect = null;
        returnFiber.effectTag |= Incomplete;
      }
      if (__DEV__ && ReactFiberInstrumentation.debugTool) {
        ReactFiberInstrumentation.debugTool.onCompleteWork(workInProgress);
      }
      if (siblingFiber !== null) {
        // 
        return siblingFiber;
      } else if (returnFiber !== null) {
        // 
        workInProgress = returnFiber;
        continue;
      } else {
        return null;
      }
    }
  }
  // 
  // 
  // 
  return null;
}
function performUnitOfWork(workInProgress: Fiber): Fiber | null {