Duplicate <meta name="description">

When my `index.html` already contains a `<meta name="description">`, if I add the following in prerender:

```javascript
head: {
  elements: new Set([
    { type: 'meta', props: { name: 'description', content: 'XXX' } }
  ]),
}
```

The final returned HTML will contain two `<meta name="description">` tags.