[Lord Carnifex]

My space opera game had dueling common in the Imperial fleet, marines, and legion. Mostly because of a "survival of the fittest" and "keep them sharp" attitude, although the duels are very rarely to the death, most being to the submission.

Among the enlisted ranks, fisticuffs are more common. Zero-G aikido is a common sport in the fleet, and a lot of duels between enlisted spacers is done that way. The Imperial marines, especially the battlesuit and assault troops practice a particularily brutal cross of juijutsu and dagger fighting: in duels, the knives usually don't come out unless the conflict is particularily serious. The Dagger Teams (elitie commandos) duke it out in zero G with daggers.

Officers in the fleet are usually assumed to be of a higher social class, and their duels are assumed to be more refined. Zero-G Aikido is used here, too and they do a fair amount of smallsword fencing, both in 1 G and zero G environments.

Fleet-Rank officers (silver star commanders, commodores, and admirals) are generally less physical. They actually tend to duel via various 3 dimensional and n dimensional games and simulations. 2D chess is relatively unknown, but they'd play it if they knew about it.

Duels, like I said, are very rarely to the death and are done with bare hands or with blunted weapons. They're mostly seen as a way to resolve conflicts, deal with grudges, and blow off steam. The overall philosophy is that soldiers/spacers should be competitive and aggressive. They succeed in a duel (the theory goes) by faggression, courage, and skill; part of the point is not to decide who is right, but who is the better soldier/marine/spacer/officer in a climate that encourages excellence, skill, and aggression.

Very rarely are duels fought to the death. Duels to the death happen where other navies might use courts martial: accusations of cowardice, insubordination, incompetence, and so on. Here again the aim is not so much to find who was right as much as it is to see who is the most skilled and aggressive (and thus the better fighter).

ETA: The prank wars between the cadets at the Imperial Intelligence Academy are the stuff of inter-stellar legend. Most of them carry on the tradition after they graduate. Rumors that Operations Directorate officers engage in duels of assassination are stringently denied, however.