In the Gears of War series (2 & 3 specifically, as the original doesn't feature this concept), if two players are each playing under a different difficulty setting, then achievements-wise the game will only register the lowest difficulty. This is so one player for example can't play on the highest difficulty setting (Insane) with another on the easiest, and then allow the second player to do most of the work by more easily killing the opposition. This also carries over to the Stars system in Gears of War: Judgment.
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