The issue of metagaming as it pertains to Chaos is, in my experience, unique to the site. I've not traveled the RP world as much as some, but I've been around a bit, and it's never been a hot button issue. In most cases, metagaming is, if not outright forbidden, strongly discouraged.
Chaos, on the other hand, has no such prohibitions, barring certain very specific instances. This is something of a mixed blessing. On the one hand, it opens up certain Force powers that are extremely hard to pull off in a more restrictive environment. On the other hand, we get stuff like the well publicized feud between the GE and SJO.
It's worth noting that, for the most part, metagaming is benign. It's nearly impossible to separate it from RP completely. The majority of threads simply wouldn't happen if the authors didn't know where to throw their characters. It would be nearly impossible to plan threads, or collaborate with other writers. Even the most restrictive sites acknowledge metagaming's usefulness in this capacity.
The salt comes almost entirely from the minority of cases in which someone uses, or is suspected of using, OOC knowledge for in character advantages. It's an accusation that's easy to use and hard to disprove conclusively, so it's often the first arrow in the quiver of someone looking to smear someone they don't like. People who actually do metagame maliciously often know this, and know that the situation can sometimes be turned around by painting the accusation as libel.
In dueling, metagaming is very much a double edged sword.
Dueling etiquette relies heavily on knowing both what you're doing, and what your opponent is doing, often more clearly than your character possibly could. Though you might be confused in character, as the writer, it's your responsibility to keep careful track of what's going on. You will be tempted to transfer that knowledge to your character. There's a right way and a wrong way to go about doing that.
Certain powers, such as precognition, or certain forms of empathic knowledge, require metagaming. Just about every Force user relies on them to some extent in a fight. The trick is to know what's acceptable and what's not.
In the case of precognition, it's highly situational. Saving your ass from getting one shotted at the start of the duel usually won't be viewed askance, so long as it's done right. Notice the done right part. A big part of that is not being an asshole. Narrowly avoiding the shot by the skin of your teeth is one thing. Treating your opponent like a blithering idiot is quite another.
That's really the key here. If you have legitimate grounds to use precognition, don't be an ass about it. If your character is an ass, by all means, write them correctly, but use your head. You're not trying to tick your opponent off out of character. If you get the salt flowing, you run a very real risk of getting reported for moves like that. And if you go around acting like a raging dick, the judge reviewing your moves is automatically going to be disinclined to like you. A good judge won't let that stop them from reviewing things fairly, but it still won't do you any favors.
There's also a variant of this in where the attack is dodged completely by accident. Your character didn't know that they were going to get hit, but they also didn't see that rock, and would you look at that, they just tripped. This is a technique best used sparingly. I've seen a few people who could pull it off and make it so funny, their opponent really didn't mind, but they were also careful to leave their characters vulnerable for a beat or two after the fall to balance things out.
Contrast this with a duelist who magically knows where every bullet is coming from, who can sense every danger source before it's actually dangerous. This fellow is often recognized by their constant smug internal monologue, or by their tendency to use bicycle kicks in a fight. You're not going to hurt them, because they know what you're going to do before you do.
Don't be that guy. Seriously, don't. Next to the guy who refuses to take damage because his armor protects him from everything up to and including the heat death of the universe, they're the most annoying archetype in RP.
Another common form of metagaming used in dueling relates to knowing how your opponent is feeling at any given moment. Note that I said feeling, not thinking. Unless you're a badass mentalist, you should never claim to know someone's thoughts in a duel, and even if you are a badass mentalist, it's best to be extremely careful. Knowing how your opponent is feeling, on the other hand, is usually okay, so long as you A: have a reasonable IC basis for figuring it out and B: don't treat it like a superpower.
This is used almost entirely to add flavor to a duel. If you notice your opponent is having a good time, and you're having a good time, you might share a moment of unbridled joy. If you see they're getting ticked off, maybe that gives you a bit of satisfaction. If you see they're getting annoyed, maybe you stick your tongue out or something to tick them off a little more. Stuff like that is usually fine. It adds some texture to the fight, and makes it more fun to read.
On the flip side, this can be abused if you take it too far. You're not trying to crawl into their head to psychoanalyze them. Most people find that sort of action really annoying, so it's best to stay away from it if you can help it.
One aspect that tends to get overlooked when we talk about metagaming is OOC interaction in a duel.
It's common courtesy to have an open dialogue between two duelists. Even the best fighters make mistakes, or write a post that's not as clear as it could be, or just don't get what their opponent is doing. Talking about things in a constructive and civil manner behind the scenes can really elevate a duel to the next level. If you strike up a rapport with the other writer, you can make things ten times more awesome, because you're both on the same page, and you can really flex your creative muscles.
In this instance, the OOC interaction is having a clear impact on the IC actions of the duel, but the effect is overwhelmingly positive. It just makes things more fun. Rather than getting snippy and being passive aggressive because some detail was missed, if you have a polite word with your opponent, they can turn that mistake into comedy gold. Maybe you want to try something that you know is a little shady, but would be outrageously epic. If they're willing to work with you, you might be able to pull it off without any hurt feelings.
On Chaos, my experience is that OOC interaction almost invariably runs in the other direction. Now that duel results don't directly impact the outcome of invasions, things have died down a little bit, but in the bad old days, if you managed to beat the brakes off an opponent, they would often resort to OOC pressure to browbeat you into backing off.
This is not okay. This is not even remotely okay. It doesn't matter how badly you're losing, trying to use OOC pressure to guilt an opponent into backing off is an act of low desperation. As is pitching a hissy fit, or threatening to retaliate against their other characters, or any one of a dozen dirty tricks I've seen and heard of. Bottom line, if you're losing, do so with grace.
Something else that was common in the bad old days was faction leadership pressuring duelists to refuse to take a hit, no matter how well earned. After all, taking a hit meant you were letting your faction down. This came to a head in one particularly memorable invasion, when two duelists for a faction absolutely refused to take any sort of damage whatsoever. Their opponents, getting frustrated, started dogpiling them, until there were six or seven master level characters unleashing everything they had, just to get them to acknowledge so much as a scratch.
Fortunately, things really have gotten better, but that sort of metagaming is still present. Bottom line, if you're losing, lose with grace. Don't try to make your opponent's life miserable just because they got the better of you.
So that just about covers metagaming in duels. If you can think of something I missed, or if you have questions or comments, the comments section and my inbox are open.