title: Return only unique values on form dropdown symfony 3.4
body: <p>I've made a form that submits the parent organisations of an individual user. The database table the entity is linked to contains multiple entries for the organisation(as there are multiple users associated with each organisation) and I only want to return one instance of the organisations name rather than 20 duplicates. </p>
  
  <p>I have heard there are functions such as distinct() or findOneOrNull but I am not sure how to implement them.</p>
  
  <p>Here is the code from the form:</p>
  
  <pre><code>        -&gt;add(
              'userParent',
              EntityType::class,
              [   
  
                  'class' =&gt; UserParent::class,
                  'choice_label' =&gt; function ($parents) {
                      return $parents-&gt;getParent()-&gt;getName();
                  }
              ]
          )               
  </code></pre>