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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 (
            <Fragment>
              <Suspense
                maxDuration={1000}
                fallback={<Text text="Loading..." />}>
                <ConcurrentMode>
                  <UpdatingText ref={text1} initialText="Async: 1">
                    {text => (
                      <Fragment>
                        <Text text="Before" />
                        <AsyncText text={text} />
                        <Text text="After" />
                      </Fragment>
                    )}
                  </UpdatingText>
                </ConcurrentMode>
              </Suspense>
              <ConcurrentMode>
                <UpdatingText ref={text2} initialText="Sync: 1">
                  {text => (
                    <Fragment>
                      <Text text="Before" />
                      <Text text={text} />
                      <Text text="After" />
                    </Fragment>
                  )}
                </UpdatingText>
              </ConcurrentMode>
            </Fragment>