Bad Manners in duels is very much like bad manners in real life. First of all, don't just try to be a jack*** to your opponent. If, when they lose, you don't need to say gg if it wasn't, but theres no need to rub it in their faces (i.e.: z0mg r3m4k3 ch4r plxx tyty). And if you lose, don't say they spend all their time playing bnet and that they are an emo freak who should burn in hell. (Sorry, I just really hate when people do this type of stuff to me, and I don't think other people like it either).
While dueling, pretty much doing stuff to get a very unfair advantage is considered bm. Like, against a character with no teleportation ability, slowing them to less than a walk while they are running is usually considered bm. Against sorceresses, if you get 95% resistances, and 40% absorb, usually they will do little to no damage on you. 85% resistances and 20% absorb is usually fine with most people, and I personally am fine with 95% res and 20% absorb.
Complaining that a main skill or strategy is illegal is generally considered bm. For example, I hate smite, and think its the most broken skill in the game, but I don't yell that they have to use zeal or something. Also, saying that playing defensively is bm is also kinda annoying. A smiter told my rabies fury wolf that I'm not allowed to run and then shift fury - I'm supposed to click on him and hold, then just wait until he mauls me.
Just try not to get the other person angry. Its a lot less fun dueling a dick than dueling a nice guy.