Description:
------------
When using prepared statements with named parameters that repeats in the statement work fine in emulated prepared statements, but doesn't work in non-emulated mode.

The test script is a simple way to reproduce it.

Test script:
---------------
<?php

$pdo = new PDO('mysql:unix_socket=/var/run/mysqld/mysqld.sock;dbname=test;charset=utf8', 'test', 'test');
$pdo->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
$pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

$stmt = $pdo->prepare('SELECT :a = :a');
$stmt->bindValue(':a', 1, PDO::PARAM_INT);
$stmt->execute();

var_dump($stmt->fetchAll(PDO::FETCH_COLUMN));


Expected result:
----------------
array(1) {
  [0]=>
  string(1) "1"
}

Actual result:
--------------
PHP Fatal error:  Uncaught exception 'PDOException' with message 'SQLSTATE[HY093]: Invalid parameter number' in /home/vagrant/shared/test.php:10
Stack trace:
#0 /home/vagrant/shared/test.php(10): PDOStatement->execute()
#1 {main}
  thrown in /home/vagrant/shared/test.php on line 10