When wizards are dueling, if they both know how to cast the Killing Curse, then what is the meaning of one being more powerful than the other?
Specifically, we are repeatedly told how powerful Voldemort is. What exactly does this mean? Except for his Horcruxes, he should be no more advantaged in a duel.
PS: This question is not a duplicate. Its essence is that being able to cast a killing curse is the epitome of magical power in duels, and so Voldemort should not have become anybody special.
There are several things that determine a wizard's ability. Since you specified duels, here are a few factors that would make one wizard superior to another in a duel:
Repertoire. When Harry fought Draco Malfoy in The Half-Blood Prince, he used a spell he'd just learned: Sectumsempra. Draco didn't know what it did (though to be fair, neither did Harry), which made it harder to defend against.
Reflexes. During the same Harry/Draco fight, one of Draco's spells "missed Harry by inches." This shows that it's possible to dodge some spells simply by not being in their path.
Willpower. Some spells, like the Killing Curse, require a certain force of will to make them work-- you have to want to kill your opponent. Voldemort clearly has this, most others likely don't.
Training. This is a bit of a catch-all, but we have plenty of examples that show what being good at magic means. Spells can go insufficiently right (Harry's first attempts at a Patronus in The Prisoner of Azkaban are little more than puffs of white smoke) or flat-out wrong (Neville transplants his own ears onto a cactus in The Goblet of Fire). One wizard's spells could simply overpower another's.
Bonus: Reputation. There are also instances of someone gaining a reputation as a skilled witch or wizard without much evidence to support it. Everyone "knows" Gilderoy Lockhart is a great wizard, despite being completely incompetent at everything but a Memory Charm. Everyone "knows" Sirius Black is extremely dangerous, despite being nobody special in terms of magical skill. While Voldemort is shown to be a powerful wizard by any objective measure, it's also possible his reputation exceeds his actual skill.
POS has the best answer to this question, though its universe is not the same as canon. In POS, people can’t just keep casting spells. Most people will get completely exhausted if they cast two killing spells in a row, or keep holding up a Protego (especially in the face of attacks). I think canon is plain dumb in regards to magical power.
