Double Kill

From Halopedia, the Halo wiki

Were you looking for the Halo 2 (Windows Vista) achievement of the same name?
H3 Double Kill.png

The Double Kill medal is awarded for killing two opponents within four seconds of each other in standard Multiplayer games (and Halo: Spartan Assault) and within less time in campaign and Firefight. In Halo 4 and Halo 5: Guardians multiplayer and in Spartan Ops the kills need only be within 4.5 seconds of each other.

The medal was originally represented by two blue stars in a yellow circle. In Halo 4 and Halo 5: Guardians it is represented by two white stars on a navy blue circle, rimmed with light blue and brown in Halo 4 and light blue an navy blue in Halo 5: Guardians. The medal is monochrome and imposed on a shield in Halo: Spartan Assault and Halo: Spartan Strike. When someone attains and achieves this medal, the announcer will shout "Double Kill!" to the player as the medal appears in their HUD. In Halo: Combat Evolved due to a lack of visual medals however, the words "Double Kill!" will appear in the upper right corner of the screen.