Because this question comes up a lot.

"Which T-dolls should I make dupes of?"

"Are there any T-dolls worth making more than 1 copy of?"

And other such variations.

Well, you can get through the game without any dupes, but some are indeed, useful enough that you might want more than 1 copy of, simply because they're really useful for specific things and you would probably want more than 1 echelon with said things. Note that since this list will vary with new releases (and is oriented to EN, because the other servers will have other language resources for them to look up) then some dolls that you would think should appear here might not. If they don't, It means I've either gotten lazy, it's a recent announcement and haven't gotten to it (because I actually play on the KR server and check on EN mainly to write these, so I'm sometimes slow regarding EN server announcements), or I might not consider it worth it. Last updated 20th October 2019.

So what does it take to actually get on this list? Mostly that they provide a specific capability that other dolls cannot easily fill. Except MGs. The MGs are here just because of cheesing reasons.

So then, let's get to to it, as well as why you MIGHT want to make to own several.

Handguns

• Pretty much all 5 Star Craftable Handguns that focus on providing buffs to your echelon will find some use. They're simply really REALLY good buffers, and there's a whole lot styles and builds that want them in various combinations, but most of them also overlap. If you wish to field more than 1 such set of formations (for example, running an IWS-team and a WA2k/SVD team will both want Welrod and Calico), then you'll want lots of 5 star handguns, equal to how many you wish to deploy simultaneously. Note: Nz75 is kind of the exception. =/
• Mk23 - for... pretty much the same reason as 5 star handguns. She has the highest damage boost out of all the 4 star handguns (and actually beats the 5 stars for the most part for raw damage boost). This one is arguable these days.
• K5 - Best daylight support for MGs basically.

SMGs

• UMP45 - If you're going to have multiple instances of an SMG, you might as well have the best general purpose one. She's the most useful one for a long time (nothing really ever matches her bufftiles) and while not necessarily the best tanker (statistically some later SMGs will beat her at raw evade tanking), she's plenty good enough at it and comes with a useful skill that works on bosses. Also has a MOD3. That said, its not really a strong necessity. Actually tbh its not even entirely necessary to dupe SMGs.
• DPS SMGs are arguable because on 1 hand, stuff like Vector isn't bad? On the otherhand it's not exactly an uncommon spot either since there are other things that can take the job.

Rifles

• Rifles are very arguable because you will usually not need more than 1 of anyone unless they fill very specific roles.
• ~~IWS2000~~ - Need is greatly reduced due to M14Mod existing on EN now. Along with R93. 
• Carcano 38 - Due to the way Isomer ranking event added enemies that have dodge AND heavy armor, Grapecano is worth having a few of to kill said enemies as her skillshot doesn't miss and will annihilate said things. For normal use like if you dont intend to rank, not nearly as necessary to have more than 1. My also be less necessary depending on what EN gets before then.

Assault Rifles

• As of now, no current AR in EN that you can acquire dupes of are really worth multiple copies with a funny exception (Zas) but for a specific use. AUG is arguable due to some use in SC ranking.
• Zas M21 - She's actually not the most amazing as a combat unit (she's good, but not to the level of AR15 or AN94), but she's 1 of the best corpse drag attackers, since she can solo drag 8-1N and other dinergate-swarm heavy maps and you need more than 1 of her to efficiently corpse drag (for "its a bitch to reload just 1 unit otherwise" reasons)

Machine Guns

• At the moment, your waifu. While the Big 4 are quite nice, MGs are currently no longer deployed in colossal numbers that you will see much need for duping. That said, MG5 and PKP are the main picks if you must.

Shotguns

• Uhm... SAT-8 probably? The value of shooting for dupe shotguns is questionable simply due to the sheer expense. You can just use whoever you want for SGs for the most part because it rarely matters, but if you run into extras, she's definitely in the running for being worth the bother.
• Honestly, Shotguns are hard to dupe and you wont generally need 2 of the same ones for the most part.

General

• Your waifu. Really, if you're properly dedicating yourself to a T-doll, go raise and oath a pile of them to prove your devotion to others.

tl;dr - Good buffing Handguns and your waifu. All the others are arguable unless you intend to rank in events or aren't out yet.

But then there's the other question, SHOULD you actually make it? That's up to you. If you don't want to, then it just means you can't deploy echelons that want 2 of the same doll and work around that. Can you go through the game without ever having to? Yes, again, just means you can't deploy 2 echelons that want to use the same doll. This merely tells you if you'll get use out of it. Actually doing it is your own problem.