That is one of my pet peeves. When I go on an inspection and I see twin breakers, I have to look at the wiring diagram and/or the panel model number. Some panels like the ITE (Siemans) 20-20 panel means that it can handle 20 full size breakers only. No twins. Then the contractor usually argues with me that panels can haold up to 42 circuits. Well that is true, however the manufacture of a certain panel only wants a certain number of breakers in that panel. I wish that the manufactures all had seperate configurations for their own equipment and not let everyone elses in there. It might be so much easier.