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Lightsabers are real now. To be technical, proto-sabers are real now, but that is a linguistics debate as we are outsiders to the Star Wars universe. Here is how they should be used if they get popular enough to resurrect dueling to first "burn", if not death (hopefully this never truly happens). As a spoiler summary, it is a heavily modified version of historical swordsmanship.
Principles
The principles of swordsmanship tend to be universal. One must be patient. One must have good judgement. One must understand timing, use time of the hand and mezzo tempo concepts. One must know distance and place. Hit them at the closest target if they are positioned where an attack will be too slow. The close fight is surprisingly safe because the focus is the emitter, not the point.
Stance and footwork
The weapon should be point forward of point up in order to constrain the blade. The body is to be profiled in order to reduce target area; it can be upright or leaned. The offhand should be on the hip or behind the back if not to stabilize movement. Deep lunges in the typical Italian or French manner is a poor idea. To keep the weapon constrained, offline steps passes are best.
Defensive actions
There are only two ways to defend against these weapons as the blades will pass through each other. First is to not be there, either by not fighting in the first place, denying blade contact, or by using body voids. The latter of which is very risky without the next method of defence. The second defence is to constrain the blade, referred to as "stringere" in rapier sources. With lightsabers as they are now, constraining is to push one's blade into the opponent's to cut its length off as opposed to steel in which it pushes the blade away. This is because with rapiers, one is safe(ish) once passed the point; lightsabers, however, one is not safe until past the point, which means the fencers are to be in grappling range.
Offensive actions
Warning, no offensive action is to be taken until the fencer is past the hilt or is in the place one cannot be hit but can hit faster than the opponent. Thrusts and cuts are of equal value. Thrusts are best delivered by a mere step in. Cuts need little effort and are to be delivered from the wrist. In some sources these kind of cuts are called half cuts (namely in rapier systems), others they are called snap cuts (namely in Chinese jian sources). Another valid cut is the slice in the manner of a draw or a push akin to early sabre systems, especially if one misses a thrust as seen in many rapier systems.
Feints
Feints are viable but risky unless the fencer can gain the place.
Grappling
Fighting in close measure is safe when passed the hilt of the lightsaber. From here one can apply any close plays of disarms. It is this range which one should do most offensive actions if choosing to constrain the blade.