I buy multiples of everything. It doesn't make sense, from a practical standpoint, to buy many different tools and weapons that all do the same thing. Weapons require training, and different weapons require different training. If I'm going to invest my limited time and resources into learning a weapon, I'd rather have doubles of weapons I'm skilled with, than many different weapons, one of which I'm any good with.
For guns, I currently have 4x 4" S&W K-frames, 2x 2" J-frames, 3x Remington 870s, all with rifle sights smoothbore bbl extended magazines. I can't stand magazines, so I don't own any. I'll be getting a Garand or three one of these days, but only after I master the shotgun and it's time to move on.
For knives I'm still experimenting, I have a benchmade Stryker, Spyderco Karambit, Spyderco Endura waved, an Emerson Commander, and an Emerson Combat Karambit. I really like the Karambits, and they're fun, but they seem to require a completely different method of operation than a standard folder. I will be buying an Emerson CQC 10, a CQC 7, and a Sypderco Military, then I'll play with all of them, think on it, and decide which one to order multiples of and which ones will be traded away or go into the safe. I'm pretty sure that the Commander design is my favorite, and if it is I'll be buying a black plain edge to go with my satin combo edge. One will be used, the other sit in the safe.
So, yes, I favor buying multiples of knives.