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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

while (nextEffect !== null) {
    if (__DEV__) {
      ReactCurrentFiber.setCurrentFiber(nextEffect);
    }
    recordEffect();
    const effectTag = nextEffect.effectTag;
    if (effectTag & ContentReset) {
      commitResetTextContent(nextEffect);
    }
    if (effectTag & Ref) {
      const current = nextEffect.alternate;
      if (current !== null) {
        commitDetachRef(current);
      }
    }
    
    
    
    
    let primaryEffectTag = effectTag & (Placement | Update | Deletion);
    switch (primaryEffectTag) {
      case Placement: {
        commitPlacement(nextEffect);
        
        
        
        
        
        nextEffect.effectTag &= ~Placement;