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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

rootContainerInstance,
            currentHostContext,
            workInProgress,
          );
        }
      }
      break;
    }
    case ForwardRef:
      break;
    case SuspenseComponent: {
      const nextState = workInProgress.memoizedState;
      const prevState = current !== null ? current.memoizedState : null;
      const nextDidTimeout = nextState !== null && nextState.didTimeout;
      const prevDidTimeout = prevState !== null && prevState.didTimeout;
      if (nextDidTimeout !== prevDidTimeout) {
        
        
        workInProgress.effectTag |= Update;
      }
      break;
    }
    case Fragment:
      break;
    case Mode:
      break;
    case Profiler:
      break;
    case HostPortal:
      popHostContainer(workInProgress);