After I upgraded to WordPress 3.5.2 I noticed that featured images started to be shown twice in excerpts, once in normal size and another (new) as a thumbnail (see goo.gl/CKiLg ). I use Atahualpa theme, but the problem does not go away when I temporarily switch to Twenty Twelve theme. Never had this problem before. Any thoughts?

It's obviously not theme related but you might try different ways of configuring things. Here are some areas to consider.

• Perhaps don't use a featured image
• Double check the "don't strip these tags" settings
• Check to see if you are using manual excerpts and how they are configured
• Deactivate all of your plugins to see if that has a bearing
• etc.
• etc.