The way you know this is 100% true.

Someone who has a tank or healer job on one char will bring in a dps alt to level them up. It's always painfully obvious they are duoboxing since this is not a game you can easily play 2 chars at once, especially well, and if you think you are, you're not. The dps is horrendous since either the dps just stands there auto-attacking making the occasional move/standing in aoe/rotation is slow or the healer/tank faults on their duty when they shift attn., but no one bats an eye about it or says anything unless its the tank/heal screwing up cause they are paying too much attn. to the dps they are also playing.