将相同的值更新到数据库时出错

这是我使用 OOP 的数据库连接，请告诉我我的执行函数有什么问题，每当我用相同的值提供更新时，它都会抛出错误。

<?php

class db{

private $conn;
private $host;
private $user;
private $password;
private $dbname;
private $port;
private $debug;
function __construct($params=array())
{
    $this->conn = false;
    $this->host = "localhost";
    $this->user = "root";
    $this->password = "mysql";
    $this->dbname = "icecreams";
    $this->port = "";
    $this->debug = true;
    $this->connect();
}

function __destruct()
{
    $this->disconnect();
    // TODO: Implement __destruct() method.
}

function connect(){
    if(!$this->conn){
        try {
            $this->conn = new PDO("mysql:host=$this->host;dbname=$this->dbname",$this->user,$this->password, array(PDO::MYSQL_ATTR_INIT_COMMAND =>'SET NAMES utf8'));
                }
                catch (Exception $e){
            die('Errer :'.$e->getMessage());
        }
        if(!$this->conn){
                    $this->status_fatal = true;
                    echo 'Connection BDD failed';
                    die();
        }
        else{
            $this->status_fatal = false;
        }
    }
    return $this->conn;
}
function disconnect(){
    if($this->conn){
        $this->conn = null;
    }
}
 function execute($query){
    if(!$response = $this->conn->exec($query)){
        echo 'PDO::errorInfo()';
        echo '</br>';
        echo 'error SQL:'.$query;
        die();
    }
    return $response;
}}

如果我用不同的值更新它会更新，如果我用相同的值更新它会显示 PDO::error 信息和错误 SQL: form execte 函数。