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.

);
    return 1;
  }
  var child;
  var nextName;
  var subtreeCount = 0; 
  var nextNamePrefix = nameSoFar === '' ? SEPARATOR : nameSoFar + SUBSEPARATOR;
  if (Array.isArray(children)) {
    for (var i = 0; i < children.length; i++) {
      child = children[i];
      nextName = nextNamePrefix + getComponentKey(child, i);
      subtreeCount += traverseAllChildrenImpl(
        child,
        nextName,
        callback,
        traverseContext
      );
    }
  } else {
    var iteratorFn = getIteratorFn(children);
    if (iteratorFn) {
      var iterator = iteratorFn.call(children);
      var step;
      if (iteratorFn !== children.entries) {
        var ii = 0;
        while (!(step = iterator.next()).done) {
          child = step.value;
          nextName = nextNamePrefix + getComponentKey(child, ii++);
          subtreeCount += traverseAllChildrenImpl(
            child,