Seat clamp keeps unscrewing, what is the preferred solution?
If a screw comes loose, in most cases it is caused by not being tight enough in the first place. It may be caused by a badly tapped bolthole where vibrations might loosen the screw which is quite rare. But I tend to believe that the screw wasn't tight enough from the beginning. (In 40+ years of cycling I've experienced one slipping seatpost because of bad sizing tolerances but never a self loosening seatcrew.) 

Tightening 'quite strong' is probably not strong enough. A torque wrench, a useful tool for working on bikes, at the recommended 6Nm setting is best here. You may apply thread lock but use the sort that can be loosened. But normally it shouldn't be necessary. 

Double collars may be useful with carbon frames and/or carbon seatposts if you want to apply the clamping force to a larger surface.