Pretty Common Problem with the Duke 390
This is a very common issue with the KTM 390.
There are dozens of posts regarding customers who are experiencing the same exact issue. Additionally, there are a few YouTube video's that show the problem happening pretty consistently.
The various proposed issues related by customers seem to revolve around these theories in particular.
The bike comes with ECU programming that is too lean.
This is an innate issue with single cylinder larger bore motorcycles and can't be fixed.
A loose connector to the ECU
A dirty throttle body
A dirty fuel injector
Inadequate fuel pump pressure, faulty fuel pump
Faulty side stand switch
Faulty tip switch (kills engine when bike falls on it's side)
Here is a list of URL's from OP's on various forums.
Additionally, there are plenty of reviews from motorcycle magazines where they complain of the same exact issue.
There is a claim that the true fix is an ECU update. The justification was that the ECU had a map that was too lean.
Here is one post.
I also called a friend that works at a local KTM shop. He indicated that there is awareness to the issue and that there is a service bulletin from KTM to update the ECU as well as set the idle 100 RPM's higher.
If it's under warranty the dealership should be aware of this and be able to resolve it without an issue.
please note that I called a US dealership, KTM is notorious for not releasing service bullets internationally
I have seen this type of issues with dozens of bikes including my own. Most common fix was the ventilation hose being bound up and incorrectly routed so that when the tank was sat on the bike, the hose pinched. After riding for a few miles a negative pressure would buildup in the tank and the fuel pump couldn't overcome the vacuum.
I have also routed a breather line that was touching the exhaust and the hose melted, preventing it from being able to breath.
Thumpers and Stalling
I do not agree with the idea that single cylinders are prone to this. I have had 6 single cylinder 4-strokes. Some of them had a stalling problem, some did not. I was able to fix all of them. Issues ranged from bad fuel pumps to clogged ventilation breathers for the gas tank to air leaks or lean conditions.
I believe the thumper stalling idea is a myth that get's perpetuated. Certainly there are some models that may have an issue, just like this bike but I do not see this as a single cylinder issue so much as a KTM quality control issue in there manufacturing process.
All of the above being said I would still warm up the bike on cold mornings. The ECU will kick up the idle for a bit more oil pressure while it warms. Although not an air cooled engine parts still expand during warm up AND the oil get's thinner as the bike get's up to operating temperature. Also, I try not to drive vehicles until they are off choke. Old air cooled engine/carburetor habit that I believe still applies to more modern FI (fuel injected) vehicles.