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.

);
          }
        }
      }
    } else if (type === 'object') {
      var addendum = '';
      if (__DEV__) {
        addendum =
          ' If you meant to render a collection of children, use an array ' +
          'instead or wrap the object using createFragment(object) from the ' +
          'React add-ons.';
        if (children._isReactElement) {
          addendum =
            ' It looks like you\'re using an element created by a different ' +
            'version of React. Make sure to use only one copy of React.';
        }
        if (ReactCurrentOwner.current) {
          var name = ReactCurrentOwner.current.getName();
          if (name) {
            addendum += ' Check the render method of `' + name + '`.';
          }
        }
      }
      var childrenString = String(children);
      invariant(
        false,
        'Objects are not valid as a React child (found: %s).%s',
        childrenString === '[object Object]' ?
          'object with keys {' + Object.keys(children).join(', ') + '}' :
          childrenString,