I think I mainly duplicate types of items, and generally essentials in plain colours so that I have options to show my statement shoes, tops, jackets against. Also I have intentionally duplicated my favourite undies( sadly no longer available but I had many pairs so some surviving) and most comfortable bras so that I have the same bra in black, white and grey.
I have 2 very similar but not the same black scoop-neck 3/4 sleeve Ts ( pic 1) and intend to take them with me when replacing as the depth of the scoop is very important flattery-wise.
With different colours, one is usually worn more, but interestingly pic 2 merino tops both 16 wears. Pic 3 the merino/acrylic pinafore tunics black has 22 wears and grey only 12 wears. I was buying the grey and suddenly bought black as well due to a discount for 2. Would not have predicted such differing wears. But they are slightly different, I like black's decorative zips.
I have recently duplicated exactly my favourite soft black jacket with 108 wears on sale( pic 4) because I have worn it so much I don't know how I will manage when it wears out! I'm storing the duplicate for now.
Oh and yes, in March 2017 ( our early autumn) I bought 2 colours of cropped summer pants. Black have had 42 wears and navy 35 wears.
I think often I don't know when I buy something how much of a workhorse it will be? I didn't know with the black jacket, but I suspected because its predecessor, a soft merino jacket, died: it was worn out!

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I used to be a haphazard duplicator. I'd buy something I liked and then I'd keep looking for more things like it because I liked it so much, which always wound up being problematic.

These days, I've become far more aware of what I'm buying and why. Now, when i duplicate something, it's because i love it so much that I would like a version of it for work that I won't cry over when my job destroys it. I've had to start keeping two distinct and separate wardrobes, because I was sick of losing favorite garments to stains and rips at work.

I often duplicate underwear, nightwear, hosiery and reading glasses in same and different colours.

When I find other items in styles and qualities that I love, I like to duplicate them ( if I can afford it), always in different colours. Here are some items that are duplicated in different colours:
● 2 skirts - taupe and silver grey
● 3 jeans - blue, grey and black
● 2 capri pants - white and black
● 2 shorts - turquoise and white
● 3 rib knit crewneck pullovers - light blue, light grey and black
● 3 cable knit crewneck pullovers - pink, white and grey
● 2 sleeveless Breton pullovers - white & red and black & white
● 2 polkadot tees - dark blue & white and white & black
● 3 solid tees - hot pink, grey and black
● 2 shortsleeved crocheted tops - light blue and white
● 2 suede ballet flats - purple and turquoise
● 3 sunglasses - turquoise, white & black, and grey
● 4 knit berets - pink, purple, blue and grey
● 2 long crocheted scarves - magenta and green
● 4 pashmina style shawls - cobalt blue, mint, turquoise and offwhite
● 2 long cardigans - offwhite and light grey
● 2 long kimonos - blue and grey
● 2 braided belts - purple and turquoise
● 2 medium sized shoulder bags - dark blue and medium grey

I don't duplicate very often because I get bored easily. I am on Team Silhouette, and I generally like my items to be distinctive in design. When I do duplicate, I usually do one item in a light color and one in a dark color. The duplicates I have in my closet that have worked well:

. Two pairs of Steve Madden stilettos, one nude and one black.
. Two white V-neck tees from J. Crew. I bought the second one because the first one was wearing out after heavy wear.
. Two Nic and Zoe lightweight cardigans, which are great for layering and work for dressy as well as casual outfits. I even bought a second copy of the white colorway because the white one was wearing out. These are workhorses.

Duplicates that did not work out well:

. Two pairs of wedge sandals, one in light brown and one in black. The black ones read too dressy for most of my outfits and did not get worn.
. Two short-sleeve Anthro tees, one in black and one in dark turquoise. I tired of the design soon after buying them, and the colors were too similar to each other in that both were very dark.

The short answer for me is that I'd intentionally duplicate more often if I had room in the budget! The reality is that I have a lot of "similar" items, which seems to work for the most part. For example, I have three pairs of black ankle pants: 1. the Halogen crop flare ponte. 2. WHBM tapered ponte (excellent fabric) 3. Polished athleisure pants with lots of stretch, and zippers at the ankles. I'm very picky about silhouette, so I wear all three slightly differently.

I also buy similar but different striped tees, and black shells/layering tops. This is well and good -- different lengths and necklines allow me to go for my perfect proportion, and keep laundry issues minimal.

However...... I'm stuck in a silhouette that bores me. It's more related to buying what I can find that is conventionally flattering and affordable and practical than it is what I love. Float cardis --- I'm talking about YOU! Fortunately, kimonos are coming to the rescue for summer! I have two at the moment, and one long boho tunic/vest topper. I'd buy a third, if the right one presents itself, but they won't be exact duplicates. Several similar items can boost the variety in my wardrobe, and that's important, too!

I notice that you say you have duplicates of two black tops. For me, two black tops would count as duplicates already.

Btw, did you ever get that dark floral sundress from Sundance?