Thanks for point to the RFC, that's great for my ticket to Audiocodes :-) Still we do not have any possibilities on our side. Audiocodes e.g. has some options to fix broken things and move on.
Am I right that the only possibility Kamailio offers is to do it with textops? Still very difficult to check with regex: - is the a c line above first m line? - if no, is the a c line after each m line? - no, not after the 2nd one - remove 2nd media, but take care on first media, is there a 3rd media? - was is part of a bundle? Yes: remove the mid from the bundle, too.
This is bad.
But I see the point: yes! It is the others fault.