I think Duo models are Chinese wholesaler model E9 aka KR808D-2 like Greensmoke.
Information is from here:
A Brief Cartridge and Atomizer Compatiblity Guide | Real eCig E-Cigarette Reviews and Information
Not much available for KR808D-2 - though it looks identical to KR808D-1 the KR808D-2 threading is different. KR808D-2 is incompatible rebrander threading. AZSmokeFree and Vaporkings.com have 1 kind of blank cartomizers. Red Dragon may have a KR808D-2 model and Greensmoke does.
LeCig is the only place that sells adapters for KR808D-2 (the adapters call it D2 and the left model listed is the battery so a D2 to D1 adapter lets you use KR808D-2 battery with KR808D-1 cartomizers (V4L, CigEasy Elite 808, plus the rubberized style model SmokelessImage, Bloogplanet, Halocigs G6, and some other KR808D-1 models pus all 2.5 to 3 ohm third party KR808D-1 cartos and standard resistance DSE901 atomizers).
So after you figure out which is right come back and tell us because maybe they switched to KR808D-1. I would like to know.