There are only two occasions when I buy more than one of the same knife. First would be is if I lost or the knife I like was filched. I have had three different CASE stockman knives taken from my tool bags over the last 40 years, and finally gave up on buying a new one.
The other would be if I find a great pattern for work. Knowing that I will eventually wear out a work knife that I use for all kinds of nasty tasks that I don't want to use my traditional to do, I buy one of these modern, screwed together pieces. I found an excellent Kershaw pattern that was on closeout (it was rumored to be too expensive to produce and didn't sell well) at about 1/2 price. Loved it. Bought two more.
Raised by Depression Era parents, I have more knives in my drawer than my father and his grandfather ever owned, even combined. Makes me feel like I have a few too many sometimes as it is.
Robert