The ledgehopped double laser, or LHDL, is a technique that can be performed with both Fox and Falco, though it is far more useful and common among Falco players due to the superiority of his laser.
The technique, popularized in Melee by Japanese Falco player Bombsoldier, involves ledgehopping by releasing the edge and jumping immediately, firing off two lasers by tapping the special button twice, and landing on the stage. Though it sounds simple in theory, the real trick is having the lasers travel at the proper height that the player wants.
If the player simply ledgehops as they normally would and tap the special button repeatedly, they will likely end up with two lasers that are extremely high and not at the same height. So the proper way to LHDL requires the player to drop from the edge a small amount further than they normally would, and to start tapping the special button immediately after jumping, on the way up, so that the player fires one laser as they pass the ledge going up, and the second as they fall back onto the stage, at the same height.
A variation of this technique, that doesn't involve ledgehopping at all, is to run off the stage, drop for a bit, and use the midair jump to reverse double laser back onto the stage. The player Westballz (in Melee) as well as DEHF (in Brawl) commonly uses this technique.
This move is semi-unsafe in Melee and Brawl if the opponent is right next to the ledge. It is less unsafe in Smash 64 due to higher shield stun as well as being less unsafe in Brawl due to the extra ledge options. In Melee lasers can be punished if a player is aware in advance, and spaced by ledge to punish it.