title: EntityType's choice_value is removing duplicate values
body: <p>Im trying to build a select html element populated with database entities. I need the labels and values in the select element to be drawn from non-default entity values, so I use the 'choice_label' and 'choice_value' options. </p>
  
  <p>The problem is that any entities with duplicate 'choice_value' (territory) values are being removed (replaced?).</p>
  
  <p>Here's my form builder:</p>
  
  <pre><code>$builder-&gt;add('country', EntityType::class, array(
      'class' =&gt; 'AcmeCheckoutBundle:Country', 
      'label' =&gt; 'Country',
      'query_builder' =&gt; function(EntityRepository $er) {
          return $er-&gt;createQueryBuilder('c')
              -&gt;orderBy('c.name', 'ASC');
      },
      'choice_label' =&gt; 'name',
      'choice_value' =&gt; 'territory'
  ));
  </code></pre>
  
  <p>This is what I expect:</p>
  
  <pre><code>&lt;select&gt;
      &lt;option value="1"&gt;United Kingdom&lt;/option&gt;
      &lt;option value="2"&gt;France&lt;/option&gt;
      &lt;option value="2"&gt;Germany&lt;/option&gt;
      &lt;option value="3"&gt;Brazil&lt;/option&gt;
      &lt;option value="3"&gt;Canada&lt;/option&gt;
  &lt;/select&gt;
  </code></pre>
  
  <p>This is what I'm getting:</p>
  
  <pre><code>&lt;select&gt;
      &lt;option value="1"&gt;United Kingdom&lt;/option&gt;
      &lt;option value="2"&gt;Germany&lt;/option&gt;
      &lt;option value="3"&gt;Canada&lt;/option&gt;
  &lt;/select&gt;
  </code></pre>
  
  <p>If I remove 'choice_value' from the form builder I get all the records but obviously with the wrong values.</p>