I have a certain pair of pants that I bought three of when I learned the brand discontinued them (stopped making women's clothes altogether, actually). I've worn through two pairs at the knees, and am down to my last pair -- which is the same cut, but a different colour and slightly different fabric. They fit a little differently too and I don't reach for them *quite* as often, but I'm definitely glad I grabbed a back-up for my back-up pair!
I also have three Patagonia flannel shirts in different colors. They redesigned the cut of their women's flannel shirt a couple years ago so that it's longer and more "curvy" now. I prefer the straighter, boxier style they used to do, so I grabbed several before they disappeared. I have enough flannels to get me through the next decade I think. And of course I still eye flannels all the time. They're a great weakness of mine.
I duplicate vintage Levis 501s when I find them. I have 3 pairs with slight differences in cut and wash; one is on its way to being retired due to threadbare patches coming through on the knees/thighs/seat. I pretty much always have an eye out for a new pair because I know I will wear them.
Oh, and I duplicate running shoes whenever I'm able to! I'm always nervous that they'll go through a redesign and the newer version will prove unwearable, so I try to have a back-up pair.
I've learned not to duplicate dressy stuff, and to buy it sparingly in the first place. I don't dress up often enough to wear my "nice" things into the ground, so having a few stellar pieces that I'll reach for year after year works for me. I try to add dressy things slowly, not in one big spree, because when I dress for a special occasion I don't want it to look like I pulled my clothes out of a time capsule. I want them to reflect me, not a particular era, if that makes sense.