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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

// one performs Sync work, rerendering the Suspense.
  if ((boundaryFiber.mode & ConcurrentMode) !== NoContext) {
    if (root === nextRoot && nextRenderExpirationTime === suspendedTime) {
      // 
      // 
      nextRoot = null;
    }
  }
  scheduleWorkToRoot(boundaryFiber, retryTime);
  if ((boundaryFiber.mode & ConcurrentMode) === NoContext) {
    // 
    // 
    // 
    scheduleWorkToRoot(sourceFiber, retryTime);
    const sourceTag = sourceFiber.tag;
    if (sourceTag === ClassComponent && sourceFiber.stateNode !== null) {
      // 
      // 
      // 
      const update = createUpdate(retryTime);
      update.tag = ForceUpdate;
      enqueueUpdate(sourceFiber, update);
    }
  }
  const rootExpirationTime = root.expirationTime;
  if (rootExpirationTime !== NoWork) {
    requestWork(root, rootExpirationTime);
  }
}
function scheduleWorkToRoot(fiber: Fiber, expirationTime): FiberRoot | null {