Kunstmaan / KunstmaanBundlesCMS

src/Kunstmaan/FormBundle/DependencyInjection/Configuration.php

<?php

namespace Kunstmaan\FormBundle\DependencyInjection;

use Symfony\Component\Config\Definition\Builder\TreeBuilder;
use Symfony\Component\Config\Definition\ConfigurationInterface;

/**
 * This is the class that validates and merges configuration from your app/config files
 *
 * To learn more see {@link http://symfony.com/doc/current/cookbook/bundles/extension.html#cookbook-bundles-extension-config-class}
 */
class Configuration implements ConfigurationInterface

This class seems to be duplicated in your project.

{
    /**
     * Generates the configuration tree builder.
     *
     * @return TreeBuilder The tree builder
     */
    public function getConfigTreeBuilder()
    {
        $treeBuilder = new TreeBuilder('kunstmaan_form');
        if (method_exists($treeBuilder, 'getRootNode')) {
            $rootNode = $treeBuilder->getRootNode();
        } else {
            // BC layer for symfony/config 4.1 and older
            $rootNode = $treeBuilder->root('kunstmaan_form');
        }

        $rootNode
            ->children()
                ->booleanNode('deletable_formsubmissions')->defaultFalse()->end()
            ->end()
        ;

        // Here you should define the parameters that are allowed to
        // configure your bundle. See the documentation linked above for
        // more information on that topic.
        return $treeBuilder;
    }
}