how to remove duplicate user friends display auto from mysql query

Question

hi there was a problem on fetching user friends photos which some photos are displaying more than one. How to fix this duplicates...

 $stmt = $this->db->prepare("SELECT * FROM app_photos WHERE slug = :slug");
    $stmt->execute(array(
            ':slug' => $this->slug,
            ));
        //$row = $stmt->fetch(); 
        $prows = array();
        while($row = $stmt->fetch()){ 
          $prows[] = $row;
        }