Assuming the 20 paces thing at say two feet to a pace => 80 feet range. Your target is moving, it's early in the morning and you are still dealing with a killer hangover or still pissed - that's why you are in this stupid duel.

To maximise your chances you turn side on, to minimise your profile and support your aim. You might consider crouching but that was probably bad form. You might consider using your other hand to steady your aim but that was probably bad form and crucially makes you a bigger target because you will be forced to stand square on.

I can't be bothered to look up what gravity will do to the trajectory of a ball from a flintlock and the like but I suspect the muzzle velocity wasn't too impressive and 80' or 25m is far enough to make the ball drop a bit vertically.

If you get hit, it will almost certainly be in the lower abdomen or leg. A soldier would know to aim a bit higher to try and get a upper torso hit. Your chest is a bigger target than anything else on you thanks to your lungs.

So a ball of very hot lead and other metals about 3/4" (?) across enters your side at about stomach level or at lower intestine level. The trauma is probably not enough to kill you outright. Modern firearms can often kill you with shock waves running up blood vessels to the heart and other organs. This thing hurts rather a lot and causes a lot of bleeding. You are being poisoned by the metals. Antibiotics have not been invented. Blood/fluid loss, shock, infection. Take your pick. I'm sure a doctor could get really creative here.

The general idea of a duel was to involve a lot of danger but honour could still be satisfied if everyone missed. Shooting someone at that range is very hard. It obviously happened but probably not as often as we modern lot think.

If you do get hit, you will probably die in quite a nasty, painful extended way.

I'm not a doctor nor a weapons expert and have some limited knowledge of all that stuff, some of my analysis might be complete bollocks! However, hopefully I've persuaded you that being shot by an old pistol is not a nice way to go. Probably not preferable to swords either. Sword duels (foils/epees etc) used to generally run to first blood, so provided you managed to stop the other bloke from sticking your eye and one of you got some blood out of somewhere not too critical with a slash or poke then all good.

Obviously, we can all imagine how a sword duel might end quite unpleasantly as well but remember that duels were about satisfying your honour and not actually killing your opponent. Killing them seems to have been considered an unfortunate side effect and not the actual goal.

EDIT: Sorry, I forgot whatever the nasty side effects of having your stomach or bowels leaking into the rest of you is called - that's a good possibility if you get hit.

Also, you are standing side on with your arm up so your lungs are a major likely target. A hit in the lung is a really nasty thing but I think will kill you quicker than most of the other traumas. It will obviously involve broken ribs but I'd like to know whether a flintlock of the time would be capable of penetrating a over coat plus clothes and ribs.

That is a key question I think. If clothes plus ribs etc is normally capable of routinely avoiding a shot being lethal at 80' (God willing etc) then we can be a bit more sure about how duels worked. Did gents suddenly remember to wear their whale bone corset to look their best (and add a bit more armour)? Did they fill their pockets with Bibles? If your coat has generous breast and waist pockets then leather bound books with many 100 thin pages will add a lot of armour to you when you stand side on.

Sadly, if your oppo gets a headshot in then you are fucked, regardless and that will probably be quick unless you are really unlucky.