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.