opencart duplicates queries...
so... go to site.dir/system/library/db.php
and replace

public function query($sql) {
return $this->adaptor->query($sql);
}

for

public function query($sql) {
$key = md5($sql);
if (empty($this->q) ) $this->q = [];
if (!isset($this->q[$key])) {
$this->q[$key] = $this->adaptor->query($sql);
}
$this->q[$key]
}