Duncan did not come from nothing
While it might be a feel good story about a random hedge knight being skilled enough to be a lord commander of the kingsguard, Ser Duncan did not actually come from nothing with regards to his skill as knight. He was trained by one of the best fighters in the realm.
From a short snippet and rewatch of other GOT materials, Ser Arlan was considered one of the top tier fighters in the realm, not the best of his time, but among the top, something like Bronn/Brienne during GOT or better.
Baelor breakspear was considered one of the best at jousting, yet Ser Arlan could tank multiple lances despite having worse equipment and being older with less training. He even managed to unhorse high lords. Almost all high lords who actively joust in GOT and HOTD are incredibly good that very few lowborn knights could actually compete. And the grey lion was considered a good fighter by the fact that even Maekar knew him for his skills.
And this was just jousting, he survived multiple wars and taught Duncan much more about swordfighting than jousting, suggesting he might be way better with a sword than a lance.
The fact that Ser Arlan could reach high noble level with cheap armor and minimal training means he was possibly top 20 or even top 10 in the realm at the time. Sure Duncan was good but had he trained under anyone else, he might not be as good a fighter as he was.
