php pdo prepare repetitive variables

The simple answer is: You can’t. PDO uses an abstraction for prepared statements which has some limitations. Unfortunately this is one, you have to work-around using something like

$query = "UPDATE users SET firstname = :name1 WHERE firstname = :name2";
$stmt = $dbh -> prepare($query);
$stmt -> execute(array(":name1" => "Jackie", ":name2" => "Jackie"));

In certain cases, such as emulated prepared statements with some versions of the PDO/MySQL driver, repeated named parameters are supported; however, this shouldn’t be relied upon, as it’s brittle (it can make upgrades require more work, for example).

If you want to support multiple appearances of a named parameter, you can always extend PDO and PDOStatement (by classical inheritance or by composition), or just PDOStatement and set your class as the statement class by setting the PDO::ATTR_STATEMENT_CLASS attribute. The extended PDOStatement (or PDO::prepare) could extract the named parameters, look for repeats and automatically generate replacements. It would also record these duplicates. The bind and execute methods, when passed a named parameter, would test whether the parameter is repeated and bind the value to each replacement parameter.

Note: the following example is untested and likely has bugs (some related to statement parsing are noted in code comments).

class PDO_multiNamed extends PDO {
    function prepare($stmt) {
        $params = array_count_values($this->_extractNamedParams());
        # get just named parameters that are repeated
        $repeated = array_filter($params, function ($count) { return $count > 1; });
        # start suffixes at 0
        $suffixes = array_map(function ($x) {return 0;}, $repeated);
        /* Replace repeated named parameters. Doesn't properly parse statement,
         * so may replacement portions of the string that it shouldn't. Proper
         * implementation left as an exercise for the reader.
         *
         * $param only contains identifier characters, so no need to escape it
         */
        $stmt = preg_replace_callback(
            '/(?:' . implode('|', array_keys($repeated)) . ')(?=\W)/', 
            function ($matches) use (&$suffixes) {
                return $matches[0] . '_' . $suffixes[$matches[0]]++;
            }, $stmt);
        $this->prepare($stmt, 
                       array(
                           PDO::ATTR_STATEMENT_CLASS => array('PDOStatement_multiNamed', array($repeated)))
            );
    }

    protected function _extractNamedParams() {
        /* Not actually sufficient to parse named parameters, but it's a start.
         * Proper implementation left as an exercise.
         */
        preg_match_all('/:\w+/', $stmt, $params);
        return $params[0];
    }
}

class PDOStatement_multiNamed extends PDOStatement {
    protected $_namedRepeats;

    function __construct($repeated) {
        # PDOStatement::__construct doesn't like to be called.
        //parent::__construct();
        $this->_namedRepeats = $repeated;
    }

    /* 0 may not be an appropriate default for $length, but an examination of
     * ext/pdo/pdo_stmt.c suggests it should work. Alternatively, leave off the
     * last two arguments and rely on PHP's implicit variadic function feature.
     */
    function bindParam($param, &$var, $data_type=PDO::PARAM_STR, $length=0, $driver_options=array()) {
        return $this->_bind(__FUNCTION__, $param, func_get_args());
    }

    function bindValue($param, $var, $data_type=PDO::PARAM_STR) {
        return $this->_bind(__FUNCTION__, $param, func_get_args());
    }

    function execute($input_parameters=NULL) {
        if ($input_parameters) {
            $params = array();
            # could be replaced by array_map_concat, if it existed
            foreach ($input_parameters as $name => $val) {
                if (isset($this->_namedRepeats[$param])) {
                    for ($i=0; $i < $this->_namedRepeats[$param], ++$i) {
                        $params["{$name}_{$i}"] = $val;
                    }
                } else {
                    $params[$name] = $val;
                }
            }
            return parent::execute($params);
        } else {
            return parent::execute();
        }
    }

    protected function _bind($method, $param, $args) {
        if (isset($this->_namedRepeats[$param])) {
            $result = TRUE;
            for ($i=0; $i < $this->_namedRepeats[$param], ++$i) {
                $args[0] = "{$param}_{$i}";
                # should this return early if the call fails?
                $result &= call_user_func_array("parent::$method", $args);
            }
            return $result;
        } else {
            return call_user_func_array("parent::$method", $args);
        }
    }
}