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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

return;
    }
    
    if (stashedWorkInProgressProperties === null) {
      
      warningWithoutStack(
        false,
        'Could not replay rendering after an error. This is likely a bug in React. ' +
          'Please file an issue.',
      );
      return;
    }
    assignFiberPropertiesInDEV(
      failedUnitOfWork,
      stashedWorkInProgressProperties,
    );
    switch (failedUnitOfWork.tag) {
      case HostRoot:
        popHostContainer(failedUnitOfWork);
        popTopLevelLegacyContextObject(failedUnitOfWork);
        break;
      case HostComponent:
        popHostContext(failedUnitOfWork);
        break;
      case ClassComponent: {
        const Component = failedUnitOfWork.type;
        if (isLegacyContextProvider(Component)) {