Thanks for your hint, but I searched again for double OG tags – without success.
For me it seems to be tricky (but I’m not a expert in this), so I’ll try to figure out next weekend which dependencies exist between these lines of code and the behaviour of my site links in social networks.
Anyway, currently I switched Open Graph off in Yoast and now Google+ works fine, but then Facebook pulls the content from each post and not the description … ??
If I find a solution I’ll post it here.