title: Adding <slot> in repeating content region
body: <p>I have a menu component which, simplistically, takes in an prop with an array of options and renders an item in the menu for each one. I wanted to be able to customise the markup that was inside each of the menu items depending on the use-case so I used the  placeholder inside the menu item element.</p>
  
  <p>You can see an example of this in this <a href="https://jsfiddle.net/ugf8sn83/" rel="noreferrer">fiddle</a>.</p>
  
  <pre><code>const Menu = {
      template: `
          &lt;ul&gt;
              &lt;li v-for="item in options" :class="item.colour"&gt;
              &lt;slot&gt;&lt;/slot&gt;
              &lt;span class="label"&gt;{{item.name}}&lt;/span&gt;
            &lt;/li&gt;
          &lt;/ul&gt;
      `,
      data: () =&gt; {
          return {
              options: [
                  { name: 'one', colour: 'red' },
                  { name: 'two', colour: 'green' },
                  { name: 'three', colour: 'blue' },
                  { name: 'four', colour: 'yellow' }
              ]
         };
      }
  };
  
  
  const app = new Vue({
      components: {
          custommenu: Menu,
        },
      template: `&lt;custommenu&gt;&lt;span class="colour"&gt;&lt;/span&gt;&lt;/custommenu&gt;`
  });
  
  app.$mount('#app');
  </code></pre>
  
  <p>This worked fine on v1 of Vue.JS but after upgrading to v2 I'm seeing the error "Duplicate presence of slot "default" found in the same render tree - this will likely cause render errors. "</p>
  
  <p>Is this something that is possible in v2 or is there an alternative way to achieve the same thing?</p>