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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

value = new Error(
      (getComponentName(sourceFiber.type) || 'A React component') +
        ' suspended while rendering, but no fallback UI was specified.\n' +
        '\n' +
        'Add a <Suspense fallback=...> component higher in the tree to ' +
        'provide a loading indicator or placeholder to display.' +
        getStackByFiberInDevAndProd(sourceFiber),
    );
  }
  
  
  
  renderDidError();
  value = createCapturedValue(value, sourceFiber);
  let workInProgress = returnFiber;
  do {
    switch (workInProgress.tag) {
      case HostRoot: {
        const errorInfo = value;
        workInProgress.effectTag |= ShouldCapture;
        workInProgress.expirationTime = renderExpirationTime;
        const update = createRootErrorUpdate(
          workInProgress,
          errorInfo,
          renderExpirationTime,
        );
        enqueueCapturedUpdate(workInProgress, update);
        return;
      }
      case ClassComponent: