What is the difference between duelist and an assassin?

A duelist should have a better chance 1v1, an assassin should be able to kill a low def target quickly even in a teamfight.

Assassins are more burst oriented, have better escapes for destroying a carry and getting the fk outta there.

Duelists/skirmishers (yas, riven, yi, xin, fiora, jax) have better sustained damage and usually some form of harder CC to give them the advantage in drawn out 1v1/2v2 skirmishes.

For example talon's defensive options are his invis/parkour to get away fast after bursting someone

where jax/fiora/xin have defensive abilities that help them stay in the fight.

Duelists tend to be more scrappy: champions capable to take a 1v1 efficiently

Assassins are swift damage dealers, able to take out a low defensive target and potentially have an escape route built in...unless it's a team fight with no escape.