minor symfony#17604 [2.7] [2.8] [3.0] [minor fix] remove duplicated value (HeahDude)

This PR was merged into the 2.7 branch.

Discussion
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[2.7] [2.8] [3.0] [minor fix] remove duplicated value

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | -

Commits
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d15c34a remove duplicated value

Author: fabpot

src/Symfony/Component/Form/Extension/Core/Type/ChoiceType.php

@@ -407,7 +407,7 @@ public function configureOptions(OptionsResolver $resolver)
         $resolver->setAllowedTypes('choice_value', array('null', 'callable', 'string', 'Symfony\Component\PropertyAccess\PropertyPath'));
         $resolver->setAllowedTypes('choice_attr', array('null', 'array', 'callable', 'string', 'Symfony\Component\PropertyAccess\PropertyPath'));
         $resolver->setAllowedTypes('preferred_choices', array('array', '\Traversable', 'callable', 'string', 'Symfony\Component\PropertyAccess\PropertyPath'));
-        $resolver->setAllowedTypes('group_by', array('null', 'array', '\Traversable', 'string', 'callable', 'string', 'Symfony\Component\PropertyAccess\PropertyPath'));
+        $resolver->setAllowedTypes('group_by', array('null', 'array', '\Traversable', 'callable', 'string', 'Symfony\Component\PropertyAccess\PropertyPath'));
     }
 
     /**