As far as I know there is almost no pre-qualification required for a private sparring session and even the qualification to be a professional is a very low bar (IIRC it is primarily a matter of eyesight, since you can't wear glasses in the ring).
You can endanger yourself (do hiking or parkour or extreme mountain-biking), but there are no legal activities to mutually endanger each other. You can't have a box fighting or chicken race to death legally. But why not?
In my understanding, it wasn't actually a given that someone would die as the result of a duel. While certainly a possibility, the poor accuracy of pistols of the day (if pistols were chosen) and the possibility of the duel being stopped by the physicians after blood was drawn (if swords were chosen) meant that honor was often deemed to be satisfied well short of anyone being killed.
It was also not uncommon for either or both combatants to simply fire their pistols into the air. Again, honor was generally deemed to be satisfied if this happened. This took some courage if you were the first one to fire into the air, as you were still required under the code to stand there and let the other guy take a shot at you if he so chose.
The pistols are not necessarily any less accurate than a modern one, in my experience. But, of course, marksmanship with handguns at further than a few paces is often difficult.
I'd guess the big danger for duelists would have been damage to internal organs or deep wounds becoming infected. Bullets could do this, as could the smallsword, a purely thrusting weapon which would have been the sword used in the 18th century. McBane (https://en.wikipedia.org/wiki/Donald_McBane) wrote of the difference between cutting and thrusting swords "the smallsword kills, yet you might take fifty cuts and not be disabled".
If McBane was telling the truth he certainly managed to get away with a large number of duels, though.
The pistols are necessarily less accurate than a modern one, as in the pistol duel traditions I've heard of in the 19th and 20th century (location does make a difference, though), the rule was to use smoothbore single-shot pistols which were less accurate than contemporary rifled pistols.
That depends on the distance and the manner of loading.
Smoothbore muskets are inaccurate, aren't they? Hence the need for 18th century armies to fire big volleys in order to get any hits.
A part of this is that the manner of loading, particularly under the stress of combat, causes variation in the powder charge. The amount of fouling increases as more shots are taken. The loose-fitting ball bounces up the barrel and will leave on slightly-varying trajectories. Hitting a man-sized target at 50 yards is difficul.
But, if you take such a musket to a modern meeting (e.g. https://www.mlagb.com/) and load it carefully with a patched ball and well-measured charge, under relaxed conditions, with the bore swabbed for a consistent amount of fouling, then they become sufficiently accurate for tight (a few inches) groups at 50 yards.
Pistols are much the same; loaded carefully, as I assume would happen before a duel, and they are very accurate. Of course, if the seconds were sloppy and used an undersized unpatched ball, and the distance was more than 10 paces, I'd expect inaccuracy.
Some rascals had their pistols rifled until within an inch or two of the muzzle so it wouldn't be noticed on a casual inspection.
As far as I know there is almost no pre-qualification required for a private sparring session and even the qualification to be a professional is a very low bar (IIRC it is primarily a matter of eyesight, since you can't wear glasses in the ring).