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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

invokeGuardedCallback(null, ref, null, null);
        if (hasCaughtError()) {
          const refError = clearCaughtError();
          captureCommitPhaseError(current, refError);
        }
      } else {
        try {
          ref(null);
        } catch (refError) {
          captureCommitPhaseError(current, refError);
        }
      }
    } else {
      ref.current = null;
    }
  }
}
function safelyCallDestroy(current, destroy) {
  if (__DEV__) {
    invokeGuardedCallback(null, destroy, null);
    if (hasCaughtError()) {
      const error = clearCaughtError();
      captureCommitPhaseError(current, error);
    }
  } else {
    try {
      destroy();