INSERT INTO – Subquery returns more than 1 row (PHP/MySQL)

I’m trying to develop a website with PHP and MySQL. Here is my PHP code I tried:

$sql = sprintf("INSERT INTO request_boms SET
                                request_list_id = %d,
                                project_version_id = %d,
                                amount = %d,
                                user = '%s',
                                timestamp = %d",
                                $requestNo, $bomID, $amount, $_SESSION["admin_user_name"], time());
$this->db_inventory->Execute($sql);

My $this->db_inventory variable is connected with an DB API. There is no issue with it. But when I execute the code up here returns me this:

Subquery returns more than 1 row
INSERT INTO request_boms SET request_list_id = 14, project_version_id = 429, amount = 1, user = 'admin', timestamp = 1607510083

I searched this issue on here (stackoverflow) but in all issues have SELECT statement in their queries. I didn’t give any SELECT statement in my INSERT INTO query. How could it be possible?

Edit (Due to comments)

Here is my Execute()

public function Execute($sql) {
    $rs = new RecordSet($sql, $this->db_type, $this->conn);
    return $rs;
}

And RecordSet

class RecordSet {
    public $rs;
    public $db_type;
    public $conn;

    public function RecordSet($sql, $db_type, $conn) {
        $this->db_type = $db_type;
        $this->conn = $conn;
        if ($this->db_type == 'sybase') {
            $rsx =@sybase_query($this->sql_escape($sql)); 
            if (!$rsx) {
                $this->queryError(sybase_get_last_message(),$sql);
            }
        } elseif ($this->db_type == 'mssql') {
            $rsx =@mssql_query($this->sql_escape($sql)); 
            if (!$rsx) {
                $this->queryError(mssql_get_last_message(),$sql);
            }
        } elseif ($this->db_type == 'odbc') {
            $rsx =@odbc_exec($this->conn, $this->sql_escape($sql)); 
            if (!$rsx) {
                $this->queryError(odbc_errormsg(),$sql);
            }
        } else {
            $rsx = mysqli_query($this->conn, $this->sql_escape($sql));
            if (!$rsx) {
                $this->queryError(@mysqli_error($this->conn),$sql);
            }
        }
        $this->rs = $rsx;
    }
}

Answer

Solved!

In my database I also have a table called request_components. And it had 2 primary key: id and request_list_id. After deleting request_list_id key from table, the problem solved. It seems to creating conflict between tables. If you had this problem, you should check your database.