I don't duplicate frequently, only over multiple seasons after I've figured out that an item works for me--and most things disappear by that time!
1) I bought a second identical pair of black harem pants from H&M and ADORE them as lounge bottoms that can also go outside or to the airport in a pinch.
2) I have two Everlane basic v-neck tees, first size S in white and size XS in muted black. Originally I had a size S in dark purple, but it got ragged looking. I definitely prefer the size S for more drape, and I'm thinking about replacing the XS muted black with a size S in true black now they have that color.
3) Lots of duplicates of my favorite bra, the Natori Feathers! I own it in emerald green as well as two each of the black and nude. Honestly it'd be most useful to duplicate again in nude, but the green color cheers me up.
4) A vintage Anthro skirt which I first bought in plum and loved so much that I duplicated in kelly green from an online reseller a few years later. But magenta fits much better into my wardrobe than kelly green does; I'm thinking about passing on the green skirt.
5) Almost forgot: BF jeans! I finally bought a pair this summer and wore them so much that I duplicated, albeit again in a different wash and size. I don't like the size up as much, but still wear them a ton.
6) Over time I've bought several pairs of AG Stevie colored skinnies, but I don't really think of those as duplications--I know that the fit works for me and each color serves a distinct purpose.
Hmm, evidently I duplicate a lot more than I thought!