A zombie which has been bitten by a zombie of different origin, which feeds on both humans and zombies of its original form.
Theorized by DroQ, and popularized by such works as "Corpses and Generals: The Zombie Civil War" and "When Zombies Attack Zombies"
The "double zombie" now moves slowly and decomposes like the "slow" zombies, and will feed on both "fast" zombies and humans.
This is possibly due to the "slow" zombie hunger for living tissue. Some theories indicate that "fast" zombies are not dead, only diseased. Their flesh, though tainted and possibly mutated, is still living human flesh.
Another condition that could lead to double zombification involves two zombies at different stages of zombie evolution. A more highly evolved strain of zombie may re-infect a less evolved zombie.