As always, you have to balance the risk of a crazy trap-setting knifeman attack with the (opportunity) cost of building additional escape routes.
A "crazy trap-setting knifeman attack" is not the only scenario that results in a blocked exit trapping people inside the building in a life-threatening situation. Think about it for a second. Imagine, for instance, that a fire breaks out for any reason. Is this unthinkable? And how do you factor in emergency response personnel trying to go up the same stairs that are the only way out of people trapped in the building?
We should not forget past lessons for no reason.
Yes, past lessons should be remembered but it looks like there is no data that would support building double staircases in reality - it's just too rare of a problem. Same reason why almost nowhere in the world requires outside fire exits like many buildings in NYC do - they are great in idea and I'm sure there are examples of them saving lives, but it's not worth making them a requirement for every building ever made.