Double Kill

Were you looking for the Halo 2 Vista achievement of the same name? The Double Kill Medal is awarded for killing two opponents within four seconds of each other. It is available in all multiplayer gametypes and has a Bungie difficulty rating of 5 out of 10.

Description
It is represented by a brass and golden circle with two blue stars in the center of the circle. When someone attains and achieves this medal, the announcer will shout "Double Kill!" to the player that received this medal. In Halo: Combat Evolved, because there are no medals, if a player kills two opponents within four seconds of each other, they will still hear the announcer, but instead of the medal they will see the words "Double Kill!" in the upper right corner of the screen. It is the most frequently earned multi-kill medal.

Trivia

 * With the Legendary update, getting two of these in one game on a Legendary Map in Halo 3 will get the player the achievement Double Double.