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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

continue siblings;
      } else {
        node.child.return = node;
        node = node.child;
      }
    }
    
    if (!(node.effectTag & Placement)) {
      
      return node.stateNode;
    }
  }
}
function commitPlacement(finishedWork: Fiber): void {
  if (!supportsMutation) {
    return;
  }
  
  const parentFiber = getHostParentFiber(finishedWork);
  
  let parent;
  let isContainer;
  switch (parentFiber.tag) {
    case HostComponent:
      parent = parentFiber.stateNode;
      isContainer = false;
      break;
    case HostRoot:
      parent = parentFiber.stateNode.containerInfo;
      isContainer = true;