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.

*
   * @param {?object} nestedChildNodes Nested child maps.
   * @return {?object} A set of child instances.
   * @internal
   */
  instantiateChildren: function(nestedChildNodes, transaction, context) {
    if (nestedChildNodes == null) {
      return null;
    }
    var childInstances = {};
    traverseAllChildren(nestedChildNodes, instantiateChild, childInstances);
    return childInstances;
  },
  /**
   * Updates the rendered children and returns a new set of children.
   *
   * @param {?object} prevChildren Previously initialized set of children.
   * @param {?object} nextChildren Flat child element maps.
   * @param {ReactReconcileTransaction} transaction
   * @param {object} context
   * @return {?object} A new set of child instances.
   * @internal
   */
  updateChildren: function(
    prevChildren,
    nextChildren,
    removedNodes,
    transaction,
    context) {