the original ak-47 was designed with a double hook trigger. why?....i don't know.
in 1959 the akm was introduced to replace the ak-47.
the akm featured a device known as a "rate reducer", "hammer retarder" or "anti bolt-bounce". this device purposefully slowed the lock time (delayed hammer fall) and allowed the bolt a few more seconds to lock into battery...thus increasing safety and increasing accuracy in full auto fire mode.
the device was engineered into the right side of the fire control group, and the right side trigger hook disappeared as it would have interfered with the rate reducing device.
in semi-auto, neither style of trigger is advantageous over the other...except to the smith in search of tuning the trigger pull. a double hook setup has 4 surfaces that must be precision matched in order that both sides function in unison. this is more dificult to accomplish than precision tuning a single hook trigger.
some folks clain a single hook setup 'twists' and causes binding. i call bullshit on that statement. one merely has to look at the finest trigger on the market, the redstararms unit, in order to disproove that theory.
the redstar trigger is light years ahead of any commie trigger and is capable of delivering pull weights, travel adjustments and most importantly, consistant feel and letoff worthy of a match rifle...and it a singlehook design.