Regarding 1v1s: If you see two players strafing with their guns and duking it out just leave them be. (however do be ready to pick off the other team's player should they win.)
Keep your eye on the chatbox. Often players will request 1v1s with someone on the other team, don't ruin their fun but be prepared to engage them after the duel.
Don't farm the noobs for damage. It is tempting but it shows maturity if you only engage them when necessary (capturing points or defending), going out of your way to kill them drives potentially good players from LD (after all do you want a grudge holder to back themselves out of a 2v2 just so you will get killed?).
Don't hurl insults at the player who kills you just accept it try to take them out next time you see them or team up with teammate and work together.
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