Just to add my first 2 cents to this forum, I got the Duo Pro from ElectronicCigarettesInc.com a couple of weeks ago (before discovering this forum), and this past week after they messed up my order for more cartridges* I started searching for alternative sources. Found this thread and was a little frustrated by the lack of a definitive answer regarding compatibility (or any other source even mentioning the Duo Pro for that matter).
Anyway I went ahead and ordered a battery and a 5 pack of cartridges from the Vapor Kings for their kr808D-1 mostly to find out if they were compatible. I received them today, and they most definitely are.
To put it straight up and unambiguously, The Electronic Cigarettes Inc Duo Pro is fully compatible with, if not identical to, the KR808D-1, at least as sold by The Vapor Kings. The battery connection is identical in every way that I can perceive. The cartridges are also identical in every way I can perceive.
* they messed up that order but all I did was e-mail them with the problem and within 30 minutes they e-mailed back telling me they would send out the right ones right away, and I got them 2 days later, so no complaints at all with their customer service. And the batteries last me a long time, so do the cartridges. And their prices are not the lowest but fully in line with most of the others I have looked at, especially when the free shipping is factored in.