It's possible this topic has been done to death, but I'm not seeing more than a couple of casual references to it searching the boards; if I'm bringing out the whip over a dead horse, I apologize

I've been musing about this a lot in the last few days, and I'm really curious what other people think.
Questing is one of my favorite parts of the game, and I love having the opportunity to hare off on little prepackaged dramas when good RP is scarce on the ground. One of the things I've noticed, though, is that most of the quests have a small issue: it doesn't make total IC sense for them to be done by multiple people. Unless the poor NPCs are stuck in some Groundhog Day scenario, they're asking multiple people to fetch the same exact book, or carry the same exact message, or whatever, over . . . and over . . . and over. How many times can the same person steal the same dagger? How often can some poor sap get investigated for the murdering the same person?
This isn't a complaint on my part, nor a request that the quests be changed, either - for starters, pretty much all of my favorite quests have this issue and I would be a sad panda if they had to be scrapped. Quests would be painfully boring if they had to be limited to events which could happen many many times! But I am curious how other people handle the obvious problem this creates re: working quest events into a character's personal canon. Do you assume they did them anyway and have them reference having done X for Y, screw the discrepancy if they meet another PC who's done the same quest? Do you treat all questing as OOC?