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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

sourceFiber.effectTag &= ~LifecycleEffectMask;
          if (sourceFiber.tag === ClassComponent) {
            const current = sourceFiber.alternate;
            if (current === null) {
              
              
              
              sourceFiber.tag = IncompleteClassComponent;
            }
          }
          
          return;
        }
        
        
        let absoluteTimeoutMs;
        if (earliestTimeoutMs === -1) {
          
          
          
          absoluteTimeoutMs = maxSigned31BitInt;
        } else {
          if (startTimeMs === -1) {