That's only because they lie about how long they last. And if you haven't noticed yet, they are a huge ripoff on parts.
Duo is actually is E9 aka KR808D-2 the near but incompatible twin of the popular KR808D-1. Both are true 3.7v slim
batteries so they deliver a slighter hotter hit than many other slim models. KR808D-1 is the most popular and most supported 2-piece slim ecig. It is the 2-piece threading all of the new blank carto styles support. The D-2 on the other hand is the fave 2-piece of ripoff rebranders who slap their own name on it and jack up the price.
VaporKings.com now has some blank KR808D-2 cartos and it's possible leCig might sell some D-2 cartos. Lecig.com also sells adapters D1 <> D2 (but their own D1 is more like a 901 - no exactly the same airflow - however the adapter is usable to use KR808D-1 and Vapor4Life cartos on a D2, or D2 cartos on a KR808D-1
If you want something stronger than D2/D1 then you need to step up to fat battery models like Ego where you can use lower resistance hotter attys and cartos, or to sturdier 3.7 volt models (2/ LR attys or cartos) which are typically box
mods and other shape mods.