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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

null,
          root,
          workInProgress,
          sourceFiber,
          pingTime,
        );
        if (enableSchedulerTracing) {
          onResolveOrReject = Schedule_tracing_wrap(onResolveOrReject);
        }
        thenable.then(onResolveOrReject, onResolveOrReject);
        
        
        
        
        
        
        
        
        if ((workInProgress.mode & ConcurrentMode) === NoEffect) {
          workInProgress.effectTag |= CallbackEffect;
          
          const nextChildren = null;
          reconcileChildren(
            sourceFiber.alternate,
            sourceFiber,
            nextChildren,
            renderExpirationTime,
          );
          sourceFiber.effectTag &= ~Incomplete;