gistfile1.js

import React from 'react/addons';

import { DragDropContext, DragSource } from 'react-dnd';
import HTML5Backend from 'react-dnd/modules/backends/HTML5';

@DragDropContext(HTML5Backend)
class App extends React.Component {
  render() {
    return(
      <div>
        {this.props.children}
      </div>
    );
  }
}

const FooSource = {
  beginDrag() { return {}; }
};

function collectFooSource(connect) {
  return {
    connectAsDragSource: connect.dragSource()
  };
}

@DragSource("foo", FooSource, collectFooSource)
class Foo extends React.Component {
  render() {
    let { connectAsDragSource } = this.props;
    
    return connectAsDragSource(<p>Test!</p>);
  }
}

React.render(<App><Foo key='1' /><Foo key='2' /></App>, document.body); // Error!