Disclaimer: not a medical professional nor have I heard of a collapsed duodenum.
I do have significant bowel issues though and from my own experience, I would guess that your collapsed duodenum is the cause of your symptoms (although you don't explain what these are), or at least part of them. Especially given that you've not said they found any evidence of disease.
As to the seriousness of it, I think how critical it is depends on how severe the collapse is. I'm assuming, like you, the collapse refers to a physical collapse of the bowel.
Depending on the severity and its impact on your life, it could require surgery to fix or you might be able to manage it conservatively going forward. And in your shoes, I would have assumed it was the reason why the scope couldn't pass through the duodenum.
Really, you should have an appointment with your consultant so that they can explain exactly what the results mean for you and what your options are.
Has an appointment not been made? If not then don't hesitate to chase it up and your gp might be helpful for this but given that your scan was a few weeks ago, I'd go straight to your consultant and their secretary and insist upon a face to face follow up with them. I would absolutely expect to have this appointment within 4-6 weeks depending on severity (although pre covid I'd have said 2 weeks).
Take a list of all questions and concerns you have, take your time in the appointment and make notes when you're there or take someone with you.
As a minimum, I'd imagine your questions to include:
What is a collapsed duodenum exactly?
Is there another term for it? (If so will help you research)
What caused it?
How serious is it?
How does it relate to your symptoms?
What are treatment options? ...Surgery, liklihood of and risks/benefits?
Can it be managed conservatively? How?
...through diet or medication?
Do you need to avoid certain food and medication as a result?
Long term prognosis for impact on your life, severity, chance of reoccurrence and how it wl be dealt with.
Is it always collapsed or the collapse happening intermittently? If so, why?
Has it been pushed by something outside to collapse? (Although you'd think this would have been spotted on ct)
To me, a collapsed duodenum suggests part of the bowel or bowel wall is collapsing inwards, causing an obstruction. Bowel obstructions are nasty. There's a risk of infection, damage to the tissue (which in very severe cases can lead to tissue death) through obstructing the blood supply etc. Obstructions are also painful.
There's a difference,however, between acute obstructions which are liable to be excruciating and are medical emergency and living with something like a stricture or narrowing of the bowel down to something like adhesions.
Living with a compromised bowel (eg stricture or narrowing) can also be miserable and can also need conservatively management. Eg diet and medication.
You might find it useful to ask about this on the Crohns and Colitis charity forums where there might be others able to advise either from personal experience of a collapsed duodenum, or, like me, those with otherwise compromised bowel function.
Don't be content to let this rest with your scan results though. While it doesnt sound like the results youve received are worrying, you should have had a follow up to discuss AND given that they're investigating symptoms and nothing else has been found, they ought to be looking into this further.
To me it sounds like no one is taking actual responsibility for organising your follow up which absolutely should have happened.
You might need to kick up a fuss to get the cogs of the nhs appointment system turning, op, but do so.