The Doppelgangers are bosses in Double Dragon II: The Revenge. They are evil clones of Billy and Jimmy Lee. Their exact origin is unknown, but they are shown to have come to life from the brothers' own shadows.
The Doppelgangers are evil clones of Billy and Jimmy Lee who challenge them as the final bosses in the original arcade version of Double Dragon II: The Revenge. They make their appearance after defeating the supposed main antagonist Willy Mackey.
Their origin is unknown, but it's evident they are not from this world as they possess diverse magical powers. They are born from Billy and Jimmy's shadows (or are their actual shadows that have come to life through magical means). Being clones, they have the same abilities as the Lee brothers, and as such, they can imitate all of their Sou-Setsu-Ken arts to perfection, meaning that they can make use of secret techniques such as their trademark "hurricane kick".
The creature is also able to project dark fireballs from its hands for ranged attacks and instantly perform finishing uppercuts, although perhaps its most devastating attack consists in it suddenly transforming back into a shadow and entering the body of its opponent (ominously laughing in the process), followed by attacking them from inside, taking a big chunk of their health and knocking them down without them being able to retaliate in any way. The only way to defend against this attack is by not making contact with the floor; as such, constantly performing hurricane kicks will greatly lower the chances that the attack triggers, as it lets the player to stay airborne for a longer time. If the Doppelganger was unable to attack for a determined period of time, it will eventually exit the host's body and continue fighting as normal.
As to what their motives are, if they are the real leaders of the Black Warriors or just an evil force that drove them to do the atrocities they did, that has never been disclosed. They could also be a symbolic way of showing the brothers overcoming their inner selves against someone who equally matches their abilities; as well as a way of dealing with their evil sides. Then again, it could just be something added by the developers as an ending surprise for the player after defeating the supposed final boss, Willy. All these theories are mere speculation, though.
In the NES port of Double Dragon II, on the "Supreme Master" difficulty setting (which is considered to be the canon of that continuity), the Doppelgangers are the penultimate bosses in the game, and after they are defeated, the player has to go through a final showdown against the Mysterious Warrior, another being that presumably also controls magical powers, hinting that he could have been involved in their creation in some way, with them just being powerful loyal servants for the brothers to contend with.
In the 2013 remake, Double Dragon II: Wander of the Dragons, the Doppelganger is fought again as the final boss, reprising its role from the arcade original.
While in the game they appear as a perfect clones of the Lee brothers, their gender is left to ambiguity (if any), as they could very well be unknown life forms that simply adopt the appearance of their opponents.