Hello Bénédicte,
Sorry for late reply to this,
I have checked in your page source, it seems like it showing the duplicate
og tag, see my screenshot :
http://screencast.com/t/wNFOdAfDZ
The bottom
og tag is come from your template file
JoomlaFolder/templates/rt_paradigm/index.php (
http://screencast.com/t/DeVoCak5lg ), when the Joomla load the file sources code from top until bottom, so the Joomla is take the second
og tag, when you click "SHARE" button in every blog post, the result will showing the same title and content.
You can remove the code from line 56 until 59
<meta property="og:url" content="<?php echo JURI::root(); ?>" />
<meta property="og:image" content="<?php echo JURI::root(); ?>templates/<?php echo $this->template; ?>/images/logo/logo-og.png" />
<meta property="og:title" content="<?php echo JFactory::getApplication()->getCfg('sitename'); ?>" />
<meta property="og:description" content="<?php echo JFactory::getApplication()->getCfg('MetaDesc'); ?>" />
After you remove and see how it goes. (Remember clear your Joomla cache)
If some of the blog post still can't show out correct the content, so you have to copy your blog post URL, paste it at here
https://developers.facebook.com/tools/debug/ , let it clear the cache from facebook.
Hope this help.