Bugs in wordpress [I think[

I wanted to redeclare functions(in codex says that i can) from wordpress/plugabble.php in my first plugin for wordpress. The problem is that when i try to active it, it give me an php error: Cannot redeclared function blabla, declared in wp-admin/plugabble.php. Why? It’s a bug?

here is are functions:

function wp_authenticate ($username, $password) {
				$strCookie=base64_encode(join(':',
					array(
						$username,
						crypt($username, 22),
						md5($password)
					)
				));

				setcookie("mnm_user",mysql_real_escape_string($username));
				setcookie("mnm_key", $strCookie);

}

function wp_logout() {
setcookie("mnm_user", "");
setcookie("mnm_key", "");
}

function wp_generate_password($length=NULL, $true=true) {
  if ($length == NULL){

  return $_POST['password'];
  }
  else{
  $chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
	if ( $special_chars )
		$chars .= '!@#$%^&*()';

	$password = '';
	for ( $i = 0; $i < $length; $i++ )
		$password .= substr($chars, wp_rand(0, strlen($chars) - 1), 1);
	return $password;

}

function wp_hash_password($password, $salt=NULL) {
    if ($salt === null) {
        $salt = substr(md5(uniqid(rand(), true)), 0, SALT_LENGTH); }
    else {
        $salt = substr($salt, 0, SALT_LENGTH);
	}
    return $salt . sha1($salt . $plainText);
}

another bug: i am using wordpress 2.8 on a site and in another 2.8.1: the bug is that i cannot “upgrade automatically” to wp 2.8.2, the page stops loading on:
Downloading update from http://wordpress.org/wordpress-2.8.2.zip.

Unpacking the update.

Cheers.
Bogdan Ungureanu