If multiple children have the same key, for example ([jsfiddle](https://jsfiddle.net/olegs/p09cweky/1/)):

``` javascript
var Hello = React.createClass({
    render: function() {
        return (<ul>
        <li key={"1"}>what</li>
        <li key={"1"}>what</li>
        </ul>
        )
    }
});
```

React prints the following warning to the console:

```
Warning: flattenChildren(...): Encountered two children with the same key, `.$1`. Child keys must be unique; when two children share a key, only the first child will be used.
```

The key referenced in the warning prepends an internal-looking '.$' to the key specified by the user. This is a bit misleading - ideally, it would mention the exact key value set by the user.

}
    }
    
    for (name in prevChildren) {
      if (prevChildren.hasOwnProperty(name) &&
          !(nextChildren && nextChildren.hasOwnProperty(name))) {
        prevChild = prevChildren[name];
        removedNodes[name] = ReactReconciler.getNativeNode(prevChild);
        ReactReconciler.unmountComponent(prevChild, false);
      }
    }
  },
  /**
   * Unmounts all rendered children. This should be used to clean up children
   * when this component is unmounted.
   *
   * @param {?object} renderedChildren Previously initialized set of children.
   * @internal
   */
  unmountChildren: function(renderedChildren, safely) {
    for (var name in renderedChildren) {
      if (renderedChildren.hasOwnProperty(name)) {
        var renderedChild = renderedChildren[name];
        ReactReconciler.unmountComponent(renderedChild, safely);
      }
    }
  },
};
module.exports = ReactChildReconciler;