title: no DTD found in Symfony2.3
body: <p>I am using Symfony2.3 _ Sonata Admin + Sonata E-commerce Bundle All installation is successfully done.</p>
  
  <p>But When I go Back-end url as login then show me :</p>
  
  <pre><code>An exception has been thrown during the rendering of a template ("[ERROR 522] Validation  failed: no DTD found ! (in n/a - line 2, column 21)
  [ERROR 1877] Element '{urn:oasis:names:tc:xliff:document:1.2}trans-unit': Duplicate key-   sequence ['sonata.invoice.references.date'] in key identity-constraint    '{urn:oasis:names:tc:xliff:document:1.2}K_unit_id'. (in /var/www/xyz.com/web/ - line 172, column 0)") in SonataUserBundle:Admin:Security/login.html.twig at line 14.
  </code></pre>
  
  <p>I don't know why ?
  I am totally confuse?
  Any one know this ?  </p>