if you check your response headers for the http://www.example.com/article/the-article-title.html request and you will see that you are getting a redirect (301 or 302 HTTP status response code) to the http://www.example.com/articledetail.php?12345 url.
this should be an internal rewrite.
this could be some incorrect mod_rewrite directives in your .htaccess or apache config or it could be done in a script that the urls are rewritten to.
your http://www.example.com/articledetail.php?12345 urls should be 301 redirected to your http://www.example.com/article/the-article-title.html urls.
also you should make sure that all your internal links and navigation use the canonical urls (http://www.example.com/article/the-article-title.html)

Try putting this into your .htaccess file

Options +FollowSymlinks
RewriteEngine on
rewritecond %{http_host} ^yoursite.com [nc]
rewriterule ^(.*)$ [yoursite.com...] [r=301,nc]

that rewrite code will help to canonicalize the domain but it won't solve the problem with query string versus directory-and-title-slug urls.

thanks, though!