There is no difference between sites or articles. The headlines in the stream or in the article/site itself, the headline two times. With 3.6.4 no problems...
With 3.6.7: NewsletterNewsletter
I've also installed simplepress. There is no problem with the headline, still only "Forum" one time. Strange...
correct with 3.6.4: www.elektroroller24.com
false with 3.6.7: http://beta.pronatur24.eu