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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

switch (status) {
            case 'pending': {
              if (listeners === null) {
                listeners = [{resolve, reject}];
                setTimeout(() => {
                  if (textResourceShouldFail) {
                    ReactTestRenderer.unstable_yield(
                      `Promise rejected [${text}]`,
                    );
                    status = 'rejected';
                    value = new Error('Failed to load: ' + text);