As with every game development question that ends with 'shouldn't be so hard, could it?' the answer is 'yes, it is actually that hard.'
Because it's not just 'did you complete this quest before?' It's 'did another character complete this quest before?' And if the answer is yes, then what do you do? If all you do is ask the question, then the quest gets skipped -- it's as if you've always had that quest completed. In the extreme case, anything that depends on that quest being completed is now completed from day one, which likely wreaks havoc on anything that listens for it: other quests, phasing NPCs, usage requirements on doors and anything else that cares about it.
So now you need to build in a new case: a special quest that you get if Another Character completed the sessionplay quest, one that's exclusive (so you can't get both it and the Original Quest at the same time), and this one acts like a detour that you take if you want the sessionplay quest to become optional if Another Character already did it. So at a minimum, in addition to 'setting a one or zero flag' (which also, by the way: not as simple as that), you're then making a new detour quest for every sessionplay in the game.
It's a good example of Game Designer Brain. It's never as simple as you want it to be, and part of the challenge is thinking through all of the various permutations that you need to make sure it works like you want it to. In addition to all the other work you have to do.
MoL