Wow. Yes, I have many duplicates, especially if I have a knife I love and find out its being discontinued. Bought a second Umnum when the original pivot was being discontinued. Multiple X-24s, and 18s. One set is for use, and has black scales. The other set have orange scales, fancy lanyards, fancy standoffs, etc, for collectability/pimping. I have 2 Rockstead Higos, because when I bought the first one, I fell completely in love, and bought a second because of how scarce they are. Still one of the most amazing items I own, in any category. Sometimes I'll end up selling the nicer one of a duplicate set, to buy something else.
To me it comes down to this: If a Glock is a good idea, a second one is just as good of an idea, assuming the financial part isn't a problem. Spares mean spare parts. It means if one is being serviced, the other one is still at the ready. It means being able to have the same tool in two places at once, for either convenience or emergency. With knives, sometimes I'm just so pleased with some new interpretation, or a tried and true design, of the oldest tool in the world, that I buy a second simply out of principle.