As I'm running the flattr plugin the excerpts on my content page are duplicated. I get the summary for each post twice. If I deactivate the plugin I still get the error. The only way fix it is to either rename the flattr plugin folder or delete it.
The only reason I figured it was the flattr plugin was because I first bulk deactivated all plugins and when that didn't work I renamed my plugin folder. Since that worked I renamed each individual plugin folder and it was when I got the flattr folder that the problem disappeared.