react.js:3640 Warning: Each child in an array or iterator should have a unique "key" prop. Check the

render: function () {
            var createItem = function (u) {
                return (<h3 key={'li_' + u.id}> {u.username + ':' + JSON.stringify(u.roles)}</h3>);
            }
            return (
                    <ul>
                        {this.state.users.map(createItem)}
                    </ul>
            );
        }