I used to but I stopped, however I'm not a collector, so the only reason I was buying doubles is in case I lost one, had it stolen, or damaged it. Over the years I've learned that I'm very unlikely to break a knife, but I've lost a couple and had a couple stolen, and I had no problems finding replacements for any of them. I've also learned that after 5-10 years my taste in knives change and while I may still have an emotional connection to an old user a NIB clone doesn't interest me like it did 10 years ago, plus fit/finish/steel on average improve over time. I still love my original 975 that looks like hell, but if I lost it today I wouldn't buy another one, too many other great options now.
These days you can find just about anything if you wait awhile, wait awhile longer and you won't pay a premium for all but the most rare knives. Want a BM975, original emerson commander, CF S90V PM2, original MT socom, LCC, Simonich Cetan (even talonite), Buck Rambo, Timberline specwar, etc. etc. you can still find them all relatively easy, in fact all but one are currently on ebay, but you might pay for it. However, if you invested that $200 wisely 10 years ago, you could easily pay the premium price and have money left over today.
If I was a collector, or taking advantage of the secondary market, I can understand keeping a NIB one in the safe and one to use, or if it was a very limited edition and I pre-ordered an extra at low prices.