maybe you should not forget
$query ="INSERT IGNORE INTO `"._DB_PREFIX_."customer_group` (`id_customer`, `id_group`) values(id_customer=". $this->context->customer->id . ",7)";
Db::getInstance()->Execute($query);
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maybe you should not forget
$query ="INSERT IGNORE INTO `"._DB_PREFIX_."customer_group` (`id_customer`, `id_group`) values(id_customer=". $this->context->customer->id . ",3)";
Db::getInstance()->Execute($query);