title: Use 2 'mainEntity' with JSON-LD
body: <p>I would like to present a book in both paperback and ebook format with JSON-LD. </p>
  
  <p>When I'm adding another <code>mainEntity</code>, the structured data testing tool finds a duplicate key:</p>
  
  <pre class="lang-html prettyprint-override"><code>            &lt;script type="application/ld+json"&gt;
              {
                    "@context": "http://schema.org",
                    "@type": "WebPage",
                    "mainEntity":{
                            "@type": "Book",
                            "author": "http://mywebsite.com/authors",
                            "bookFormat": "http://schema.org/EBook",
                            "datePublished": "2017-08-26",
                            "image": "http://mywebsite.com/images/coverImage.jpg",
                            "inLanguage": "en-US",
                            "isbn": "ebook isbn",
                            "name": "my website",
                            "numberOfPages": "200",
                            "offers": {
                              "@type": "Offer",
                              "availability": "http://schema.org/InStock",
                              "price": "30",
                              "priceCurrency": "USD"
                            },
                    "mainEntity":{
                            "@type": "Book",
                            "author": "http://mywebsite.com/authors",
                            "bookFormat": "http://schema.org/Paperback",
                            "datePublished": "2017-08-26",
                            "image": "http://mywebsite.com/images/coverImage.jpg",
                            "inLanguage": "en-US",
                            "isbn": "book isbn",
                            "name": "my website",
                            "numberOfPages": "200",
                            "offers": {
                              "@type": "Offer",
                              "availability": "http://schema.org/InStock",
                              "price": "55",
                              "priceCurrency": "USD"
                            },
                            "publisher": "Publisher name",
                          }
                  }
              &lt;/script&gt;
  </code></pre>
  
  <p>Could you please tell me the best way to implement it?</p>