It's a good question actually. I would say it's a combination of respawns knowledge, good aim, good weapon choice, and some other key tactics/positioning in order to properly apply all of those aspects together to get many as many frags as efficiently as possible. The basic principle is the same as a normal duel (do what you can to beat the opponent), except you don't have as many places to hide, and of course no armours :P
good plasma is the key, as you can spam the spawns with it, for insta 100dmg, and considering little space to doge on platforms you can easly get the kill from long range with it,
7ru3 plays this kind of duel a lot. It's more similar to 1vs1 CA aim training, without rounds so frag numbers are more even. http://www.qlranks.com/demo/view/23740