Although it is somewhat understandable someone asking this question it is basically the same as asking what brand of oil anyone is using and a pretty often ridiculed bunch of questions appearing on every motorcycle forum.
As the Dukes are being delivered with different brand tyres in different parts of the world the only solid advice imho would be, to get a rear tyre size 140 and a front tyre size 110 as these fit the rims as should (I know they come with 150 rear)
This obviously is when you desire the best riding performance of your Duke..
It also largely depends on your riding style and expectation on durability and budget.
Pirelli Diablo Rosso II on the 390 on good roads and twisties is my tyre of choice, super tyre also in the wet but won't last you that long in comparison to some other brands around.
Pirelli MT60's on my 200 as with this Duke I do a fair bit of dirt roads. great in dry dirt/gravel and tarmac also in the wet but not that great in mud.
I tend to agree with Slippy as well. Important questions here are "Was there anything wrong with the performance of the OE tires?" and "Are the OE tires available at an acceptable price point?"
If you're not trying to save money (on a KTM, haha!) or looking to improve performance, go with what you are already used to, you already have 9000km of muscle memory built into your riding. Change for change's sake is a risky venture on a well-sorted bike.