"Double melee" refers to three button combos present in Halo 2, all of which have the same effect: they allow a player to melee twice, with no delay between the attacks. The three combos are B+X+B, B+Y+B, and B+L+B, the last of which also works in Halo: Combat Evolved. If the player has a secondary weapon, they can Y between melees to switch and melee with your secondary, or double-tap Y to melee with their primary weapon again. This is useful if carrying an Energy Sword or Brute Shot, both of which have much stronger melee attacks than other weapons.