I can think of four systems that might fit the bill.
I'm basing these recommendations on the following assumptions, two of which I'm assuming are implied from the question and the rest based on my experiences running a system specifically focused on one-on-one player combat (Street Fighter: the Storytelling Game, which was centered around group/singular martial arts combat).
In order for there to be tactical diversity, you need a wealth of options, either in the ability for the player to create their own options or have a huge list of them to choose from.
Low Handling Time
I assume this is another way of saying "few die rolls" or otherwise streamlined combat mechanics. All of these systems use either no die rolls, single die rolls, or single attack-vs-defense rolls, resolving effects quickly. In addition, most importantly their mechanics are universal, meaning that it doesn't matter what the actual effect is; they're all resolved the same way. This is especially important in a game with a wide variety of powers because it's only a matter of time before two spells interact in a way the designers never intended and you have to stop the game to figure out what to do.
I've found, in my experience, that despite duels simply being one-on-one battles there are a couple of important factors to consider. The two most relevant ones are system lethality and character balance. Lethality, because if duels are particularly deadly then players are discouraged from participating in them, and character balance because if characters are physically weak or otherwise unable to defend themselves, they risk being one-shotted or otherwise kicked out of the duel for reasons beyond their control, which is exacerbated by the system lethality; not only did you lose because someone was lucky enough to pick paper when you picked rock, but now you're dead. That's no fun.
The Marvel Universe Roleplaying Game by QED Games (out of print)
The MURPG system is a resource-based RPG; players have energy pools and spend points from those pools to activate their super powers. Although a superhero game, any existing power can have the source "Magical", which automatically makes it a magic power, and there are specific magic-based powers like Mastery of Magic, Magical Travel, Summoning, and Sorcery. All actions are stones (energy points) spent vs. opponent's defense, and opponents can spend points on defense in addition to their "always-on" defenses.
The magical powers, Sorcery especially, is only limited by what the player can describe, and there are a wealth of existing powers. Clever power use is the bulk of your tactical ability, and since there is a single combat mechanic the system, in my experience, never bogged down. Selecting your powers for the turn, balancing your energy between attack and defense, and figuring out exactly what to do with your powers provided a very flexible, open-ended experience.
Since the player could use any power to defend, and had passive defenses as well, it was pretty difficult to simply splatter someone out of existence with a single roll, even if one player vastly outclassed another. The one time one of my players dueled Dr. Strange, clever power use and defensive play allowed him to hold his own for quite a while despite Strange's greater skill.
The system is also fairly non-lethal; if a player takes damage in excess of their current health, they can declare themselves to be knocked out instead.
Mutants & Masterminds by Green Ronin
D20 superheroes with extremely simple mechanics; by third edition all rolls used a single d20 versus either a defensive save or flat defensive score. Similar to MURPG, it's possible to create a magical character purely by selecting/building powers and then saying "My character's powers are magical." The power-building system, especially by third edition, is one of the most flexible I've ever seen, allowing you to come up with a wealth of different abilities.
All damage in M&M is considered non-lethal by default, and there are no "character classes" in the sense that it's possible to build a mage character with similar defenses to a more physically-oriented character, as well as use their powers to defend as well as attack. I've played a mage in M&M; once your powers are built it's a bit more rigid than MURPG but battles against non-minion opponents could be outright slugfests, requiring numerous good rolls to defeat. (3rd edition, specifically, requires you to "wear down" your opponents, stacking penalties on them until they fail a save.)
Mage: the Whatever by White Wolf
I've only played Ascension, which is like, old, n' stuff. There's a newer edition out now. I home-brewed a campaign setting using its magic system based on Magic: the Gathering, though; it benefitted from a tweak or two but otherwise worked just fine.
Mage's magic system is freeform, which allows players to describe their own effects, all of which are as strong as the number of successes you roll to activate them. In order to pull off an effect, though, you need enough dots in the proper Sphere (there are ten Spheres). Using Spheres on their own, or combining them with other Spheres, provided an infinite number of effects, but again, they were all resolved with a basic attack/activation vs. soak/defense check.
In addition to free-forming it, there are Rotes - pre-defined effects that spell out what happens and how many dots in the proper spheres it required. (For example, "Fireball: Forces 3"). To keep the "Magic" flavor I simply created a ton of rotes patterned after various Magic cards.
All Mage characters are equally "squishy" - they're normal humans, which means one good roll from the right effect can flatten one. To counter this, I created a Prime rote called Dueling Barrier that mimicked the life points from Magic; invoking this rote at the start of the duel gave a player 20 points of barrier than had to be broken down by attacks/effects, at which point the player would be quite inclined to forfeit.
and, of course,
Fate Core and/or Dresden Files by Evil Hat
Fate Core doesn't have a magic system in and of itself, but the Fate Core Toolkit has a bunch, and you can roll your own quite easily. Dresden Files does have a magic system, but I haven't personally used it. The Contest mechanics use disrupting an enemy's magic ritual in the example text. Although my experience with Fate doesn't extend to using it for magical dueling, the fact that it's both rules-and-rolls-light, that magic would be no different than any other conflict, and the open-ended nature of both the system and the narrative power of the players would make it a good choice if you needed to create your own system, since the magic rules would be whatever you needed them to be.
Fate does, however, strongly advise against killing characters, so most of the time if a player is losing a duel they can simply Concede the Conflict/Contest and take a Complication. Characters are also considered competent, and in most cases will be fairly even in terms of the amount of physical/mental stress they can take. This, coupled with the typical flow of Fate combat (creating advantages, invoking them for bonuses) provides for a fairly strategic back-and-forth that most likely won't kill you.