QuickExample.php

<?php

//Entity file 

 * @UniqueEntity(fields="email", message="Sorry, this email address is already in use.", groups={"registration"})

//Form file

public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => 'FA\BackEnd\UserBundle\Entity\User',
            'validation_groups' => ['registration']
        ));
    }