title: React props validation only fired once for each Element?
body: <p>I couldn't find the details about how/when React props validation works besides the <a href="https://facebook.github.io/react/docs/reusable-components.html#prop-validation" rel="nofollow">Reusable Components</a> doc. From my experiment, it looks like the props validation will only be fired once per Element (If I understand it correctly, an Element is an object representing a DOM node, like the JSX code we would write in the <code>render()</code> function), no matter how many times the Component is initialized and rendered. An example (and <a href="https://jsfiddle.net/Lnunus7w/1/" rel="nofollow">fiddle</a>),</p>
  
  <pre><code>var Hello = React.createClass({
    propTypes: {
      name: React.PropTypes.string.isRequired
    },
    render: function() {
      return &lt;div&gt;Hello {this.props.name}&lt;/div&gt;;
    }
  });
  
  ReactDOM.render(
    &lt;Hello /&gt;,
    document.getElementById('component-1')
  );
  
  ReactDOM.render(
    &lt;Hello /&gt;,
    document.getElementById('component-2')
  );
  </code></pre>
  
  <p>I can find a <a href="https://github.com/facebook/react/pull/2920" rel="nofollow">maybe related PR</a>, but I wonder if there is more detailed information about how that works and when the error is fired.</p>
  
  <p>A real use case that I want it to be fired every time a Component is initialized is that in tests we may need to check again it in multiple test cases, and I have no idea how the get around it. Any advice would be very appreciated. Thanks!</p>